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SUMMARY TECHNICAL REPORT 
OE THE 

NATIONAL DEEENSE RESEARCH COMMITTEE 


Manuscript and illustrations for this volume were prepared for 
publication by the Summary Reports Group of the Columbia Uni- 
versity Division of War Research under contract OEMsr-1 131 with 
the Office of Scientific Research and Development. This volume 
was printed and bound by the Columbia University Press. 

Distribution of the Summary Technical Report of NDRC has been 
made by the War and Navy Departments. Inquiries concerning the 
availability and distribution of the Summary Technical Report 
volumes and microfilmed and other reference material should be 
addressed to the War Department Library, Room lA-522, The 
Pentagon, Washington 25, D. C., or to the Office of Naval Research, 
Navy Department, Attention: Reports and Documents Section, 
Washington 25, D. C. 


Copy No. 

6 


This volume, like the seventy others of the Summary Technical 
Report of NDRC, has been written, edited, and printed under 
great pressure. Inevitably there are errors which have slipped past 
Division readers and proofreaders. There may be errors of fact not 
known at time of printing. The author has not been able to follow 
through his writing to the final page proof. 

Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these reports and sent 
to recipients of the volume. Your help will make this book more 
useful to other readers and will be of great value in preparing any 
revisions. 


SUMMARY TECHNICAL. REPORT OE DIVISION 6, NDRC 


VOLUME 10 


BASIC METHODS FOR THE 
CALIBRATION OF SONAR 
EOUIPMENT 


OFFICE OF SCIEN7 IFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 6 
JOHN T. TATE, CHIEF 


WASHINGTON, D. C., 1946 


NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative ^ 

Frank B. Jewett Navy Representative^ 

Karl T. Compton Commissioner of Patents^ 

Irvin Stewart, Executive Secretary 


^Army representatives in order of service: 
Maj. Gen. G. V. Strong Col. L. A. Denson 

Maj. Gen. R. C. Moore Col. P. R. Faymonville 

Maj. Gen. C. C. Williams Brig. Gen. E. A. Regnier 
Brig. Gen. W. A. Wood, Jr. Col. M. M. Irvine 
Col. E. A. Routheau 


-Navy representatives in order of service: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Purer 

Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 

Commodore H. A. Schade 
^Commissioners of Patents in order of service: 
Conway P. Coe Casper W. Ooms 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities of 
warfare, together with contract facilities for carrying out 
these projects and programs, and (2) to administer the tech- 
nical and scientific work of the contracts. More specifically, 
NDRC functioned by initiating research projects on re- 
quests from the Army or the Navy, or on requests from an 
allied government transmitted through the Liaison Office 
of OSRD, or on its own considered initiative as a result of 
the experience of its members. Proposals prepared by the 
Division, Panel, or Committee for research contracts for 
performance of the work involved in such projects were 
first reviewed by NDRC, and if approved, recommended to 
the Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of the 
contract, including such matters as materials, clearances, 
vouchers, patents, priorities, legal matters, and administra- 
tion of patent matters were handled by the Executive Sec- 
retary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 

Division A — Armor and Ordnance 
Division B — Bombs, Fuels, Gases, & Chemical Problems 
Division C — Communication and Transportation 
Division D Detection, Controls, and Instruments 
Division E — Patents and Inventions 


iv 


In a reorganization in the fall of 1942, twenty-three ad- 
ministrative divisions, panels, or committees were created, 
each with a chief selected on the basis of his outstanding* 
work in the particular field. The NDRC members then be- 
came a reviewing and advisory group to the Director of 
OSRD. The final organization was as follows: 

Division 1 — Ballistic Research 

Division 2 — Effects of Impact and Explosion 

Division 3^— Rocket Ordnance 

Division 4 — Ordnance Accessories 

Division 5 — New Missiles 

Division 6 — Sub-Surface Warfare 

Division 7 — Fire Control 

Division 8 — Explosives 

Division 9 — Chemistry 

Division 10 — Absorbents and Aerosols 

Division 11 — Chemical Engineering 

Division 12 — Transportation 

Division 13 — Electrical Communication 

Division 14 — Radar 

Division 15 — Radio Coordination 

Division 16 — Optics and Camouflage 

Division 1 7 — Physics 

Division 18 — War Metallurgy 

Division 19 — Miscellaneous ^ 

Applied Mathematics Panel 
Applied Psychology Panel 
Committee on Propagation 

Tropical Deterioration Administrative Committee 
Library of Congress 

68 

2015 4909.S5 



NDRC FOREWORD 


AS EVENTS of the ycai's preceding 1940 revealed more 
jC\ and more clearly the seriousness of the world 
situation, many scientists in this country came to 
realize the need of organizing scientific research for 
service in a national emergency. Recommendations 
which they made to the \Vhite House were given care- 
ful and sympathetic attention, and as a result the 
National Defense Research Committee [NDRC] was 
formed by Executive Order of the President in the 
summer of 1940. The members of NDRC, appointed 
by the President, were instructed to supplement the 
work of the Army and the Navy in the development 
of the instrumentalities of war. A year later, upon the 
establishment of the Office of Scientific Research and 
Development [OSRD], NDRC became one of its 
units. 

The Summary Technical Report of NDRC is a 
conscientious effort on the part of NDRC to sum- 
marize and evaluate its work and to present it in a 
useful and permanent form. It comprises some sev- 
enty volumes broken into groups corresponding to 
the NDRC Divisions, Panels, and Committees. 

The Summary Technical Report of each Division, 
Panel, or Committee is an integral survey of the work 
of that group. The first volume of each group’s report 
contains a summary of the report, stating the prob- 
lems presented and the philosophy of attacking them 
and summarizing the results of the research, develop- 
ment, and training activities undertaken. Some vol- 
umes may be “state of the art” treatises covering sub- 
jects to which various research groups have contrib- 
uted information. Others may contain descriptions of 
devices developed in the laboratories. A master index 
of all these divisional, panel, and committee reports 
which together constitute the Summary Technical 
Report of NDRC is contained in a separate volume, 
which also includes the index of a microfilm record 
of pertinent technical laboratory reports and refer- 
ence material. 

Some of the NDRC-sponsored researches which 
had been declassified by the end of 1945 were of suffi- 
cient popular interest that it was found desirable to 
report them in the form of monographs, such as the 
series on radar by Division 14 and the monograph on 
sampling inspection by the Applied Mathematics 
Panel. Since the material treated in them is not dupli- 


cated in the Summary Technical Report of NDRC, 
the monographs are an important part of the story of 
these aspects of NDRC research. 

In contrast to the information on radar, which is of 
widespread interest and much of which is released to 
the public, the research on subsurface warfare is 
largely classified and is of general interest to a more 
restricted group. As a consequence, the report of 
Division 6 is found almost entirely in its Summary 
Technical Report, which runs to over twenty vol- 
umes. The extent of the work of a Division cannot 
therefore be judged solely by the number of volumes 
devoted to it in the Summary Technical Report of 
NDRC: account must be taken of the monographs 
and available reports published elsewhere. 

Any great cooperative endeavor must stand or fall 
with the will and integrity of the men engaged in it. 
This fact held true for NDRC from its inception, and 
for Division 6 under the leadership of Dr. John T. 
Tate. To Dr. Tate and the men who worked with him 
—some as members of Division 6, some as representa- 
tives of the Division’s contractors— belongs the sin- 
cere gratitude of the Nation for a difficult and often 
dangerous job well done. Their efforts contributed 
significantly to the outcome of our naval operations 
during the war and richly deserved the warm response 
they received from the Navy. In addition, their con- 
tributions to the knowledge of the ocean and to the 
art of oceanographic research will assuredly speed 
peacetime investigations in this field and bring rich 
benefits to all mankind. 

The Summary Technical Report of Division 6, 
prepared under the direction of the Division Chief 
and authorized by him for publication, not only pre- 
sents the methods and results of widely varied re- 
search and development programs but is essentially 
a record of the unstinted loyal cooperation of able 
men linked in a common effort to contribute to the 
defense of their Nation. To them all we extend our 
deep appreciation. 

Vannevar Bush, Director 
Office of Scientific Research and Develofmient 

J. B. CoNANT, Chairman 
National Defense Research Committee 



FOREWORD 


O NE of the principal responsibilities placed upon 
Section C-4, later Division 6, when it was organ- 
ized in 1941 was that of developing new and improved 
methods for detecting submerged submarines. Ex- 
perience had shown that sound is the only form of 
energy which can be propagated through sea water 
with sufficient intensity and range to serve as a prac- 
tical method for detection. For this reason the Section 
initiated at once a thorough study of the acoustical 
properties of sea water as well as of methods for in- 
jecting sound energy into the water and of detecting 
its presence there. Realizing that in a study of this 
kind accurate measurement in terms of known and 
reproducible standards is essential, the Section under- 
took early in 1941 to develop a number of standard 
projectors and hydrophones covering the useful fre- 
quency ranges and capable of being accurately cali- 
brated in terms of energy output or input. Further, 
the Section undertook to develop methods for cali- 
brating these standards and methods applicable to 
the accurate testing of underwater sound gear under 
development or in production by various Govern- 
ment agencies. This activity was at first conducted 
under a contract at the Bell Telephone Laboratories, 
which established two calibrating and testing labora- 
tories, the first at Mountain Lakes, New Jersey, and 
somewhat later a second at Orlando, Florida. The 
facilities of these two laboratories were made avail- 
able to all organizations developing or manufactur- 
ing sound gear for the Navy. 

In 1942 the operation of these two stations together 
with the responsibility for certain further develop- 
ment of methods was transferred, along with many of 
the experienced personnel, from the Bell Telephone 
Laboratories to the Columbia University Division of 
War Research contract. The former organization, 
however, continued the further development and 
particularly the construction of standard instruments 
which have found wide use. From the time of trans- 
fer, the organization carrying on operations centering 
at Mountain Lakes and Orlando was known as the 


Underwater Sound Reference Laboratories and was 
under the direction of Dr. Robert S. Shankland. 

In addition to the two above-mentioned testing 
stations, various of the Division’s other contractors 
found it necessary to establish at suitable locations 
less elaborate testing laboratories to facilitate the test- 
ing of sonar devices, systems, and methods at various 
stages of their development. 

The material in this report prepared by members 
of the staff of the Underwater Sound Reference Labo- 
ratories describes the methods and procedures which, 
as a result of over four years development and ex- 
perience, the Division believes can be followed in 
establishing and operating a sound reference labora- 
tory. The possible scope of such a laboratory is indi- 
cated by including a description of its activities since 
its operation under the Columbia University Divi- 
sion of War Research. 

In addition to the persons whose names appear as 
authors of chapters or sections of this report, many 
others have made important technical contributions 
to this development. 

The four-year program covered by this report owes 
much to the continuous liaison furnished by the 
Navy. The Division expresses its appreciation for the 
most helpful and cordial support and cooperation 
received from the Office of the Coordinator of Re- 
search and Development and from the Bureau of 
Ships (940). A list of the principal Navy projects is 
furnished on page 171. 

Manufacturers producing or developing sonar 
material under Navy contracts have in many and 
various ways cooperated wholeheartedly. In particu- 
lar, members of their staffs gave freely of their time to 
the work of the Flydrophone Advisory Committee ap- 
pointed by the Office of the Coordinator in April, 
1942. On page 172 the structure and the general scope 
of this committee are outlined. 

John T. Tate 
Chief, Division 6 


vii 




PREFACE 


rr^HE PROBLEM of developing and establishing accu- 
J. rate standards and procedures tor calibration of 
underwater sound equipment required the attention 
of many laboratories operating under contract with 
Division 6, NDRC. Although most of this work was 
done originally by the Bell Telephone Laboratories 
and later by the Underwater Sound Reference Labo- 
ratories of Columbia University, a number of other 
organizations contributed to the results summarized 
in this volume. In view of the wide scope of this field, 
however, it was not possible in the limited time 
allotted for the preparation of this volume to include 
a complete treatment of all of the numerous contri- 
butions. The principal reports covering this comple- 
mentary work have been included in the bibliog- 
raphy and are preserved in the form of microfdm 
records. 

In the preparation of this volume, the emphasis 
has been placed on the experience gained by Divi- 
sion 6 laboratories in developing testing methods and 
apparatus and in carrying through an extended pro- 
gram of precision measurements on underwater 
acoustic devices. The basic principles of these meas- 
urements are develgped and systematized in close 
coordination with a description of the measuring and 
calibrating facilities of the test stations at Mountain 
Lakes, New Jersey and Orlando, Florida. It is be- 
lieved that these descriptions are sufficiently complete 
to be of material service to other groups in setting up 
and maintaining similar equipment. 


As the responsibility for preparing this volume was 
assigned by the chief of Division 6 to the staff of the 
Underwater Sound Reference Laboratories, the 
greater part of the material included is, of necessity, 
drawn from the experience of this group. The entire 
staffs of the Mountain Lakes and Orlando stations, as 
well as the staff of the New York office, have collabo- 
rated in all phases of the task of collecting and pre- 
paring the data upon which the present work depends. 
This group included: Edwin L. Carstensen, E. Dietze, 
W. Richard Elliott, Leslie L. Foldy, Frank H. Gra- 
ham, Earle C. Gregg, Jr., Erhard Hartmann, Norma 
Hartmann, F. Whlliam Hoffman, Paul F. Joly, Joseph 
B. Keller, Martin J. Klein, L. Pauline Leighton, 
Lucille Northrop, Henry Primakoff, Edward S. Rog- 
ers, Robert S. Shankland, Erwin E. Shrader, D. Ber- 
nard Simmons, Richard J. Tillman. 

Acknowledgement should also be made of contri- 
butions by Division 6 laboratories at the Massachu- 
setts Institute of Technology, Harvard University, 
University of Galifornia, and Columbia University. 
In addition, valuable assistance was received from 
the Brush Development Company, Submarine Signal 
Company, the Radio Corporation of America, and 
the service laboratories of both the United States and 
British Navies. 

R. S. Shankland 
Director, Underwater Sound 
Reference Laboratories 


ix 




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CONTENTS 


CHAPTER 

1 Introduction by Robert S. Shankland .... 

2 Operation and Application of Underwater Sound 

Devices by Leslie L. Foldy 

3 Generalized Theory of Electroacoustic Transducers 
by Leslie L. Foldy and Henry Prirnakoff 

4 Types of Acoustic Measurements by Eginhard Dietze 

5 Testing Technique by Leslie L. Foldy . 

.6 Description and Operational Procedures of the 
USRL Test Stations by Erhard Hartmann and 
Earle C. Gregg, Jr 

7 Computation from Test Data by Eginhard Dietze 

and L. Pauline Leighton 

8 Production Testing of Sonar Transducers by Erwin 

F. Shrader 

9 Acoustic Equipment Associated with Underwater 

Sound Devices: Domes and Baffles by Henry Prima- 
koff and Joseph B. Keller 

Glossary 

Bibliography 

Contract Numbers 

Service Project Numbers 

Hydrophone Advisory Committee 

Index 


PAGE 

1 


10 

18 

34 


68 

138 

149 


153 

165 

167 

170 

171 

172 

173 





Chapter 1 

INTRODUCTION 

By Robert S. Shankland 


S CIENTIFIC programs generally require accurate 
standards and techniques of measurement to in- 
sure quantitative correlation and interpretation of 
phenomena under investigation. Only when an un- 
derstanding of phenomena in quantitative terms has 
been achieved can accumulated data be effectively 
and efficiently applied to the design of new equip- 
ment, to the improvement of present designs, and to 
the prediction of results obtainable with such gear. 

The program undertaken by Section C-4, later 
Division 6 of NDRC, included studies and experi- 
mental investigations on the transmission of sound in 
ocean waters and the further development of sonar 
gear. This necessitated the provision of suitable 
practical standards and a study of measurement 
techniques. The results accomplished form the prin- 
cipal subject of this volume. 

In the development of apparatus for service use 
it is generally true that the hnal criterion is the effec- 
tiveness of the equipment under operational condi- 
tions. In the case of sonar gear, operational tests are 
not only expensive and time-consuming, but are per- 
formed at that stage in development when changes 
in design are most difficult to achieve. Hence, labo- 
ratory tests under controlled conditions directed 
toward determining design changes, which will pro- 
duce maximum effectiveness under operational con- 
ditions, are a necessary adjunct to a program of 
research and development. 

I I DEVELOPMENT OF MEASUREMENT 
AND CALIBRATION TECHNIQUES 

* Establishment of Standard 

Sound Fields 

To make the results of the several laboratories en- 
gaged in subsurface warfare research directly com- 
parable, it was necessary to reduce their test data to 
common terms. Also, as the program progressed, it 
was necessary to work continually toward higher 
standards of accuracy. The Underwater Sound Ref- 
erence Laboratories [USRL] were assigned the task 


of establishing reference levels, and of making cali- 
brated standards available to the other laboratories. 

Originally, this standardization was based upon 
the characteristics of hydrophones (developed in co- 
operation with the Bell Telephone Laboratories, Inc. 
[BTL]), whose absolute calibrations could be ob- 
tained from their design or by comparison with fairly 
well established standards for air acoustics. Later, it 
was found that the reciprocity method of calibration 
provided improved accuracy and a simplified pro- 
cedure. 

The reciprocity method of calibration had been 
suggested by Ballantine and MacLean,”i’^" but had 
received little attention until it was applied to the 
establishment of fields in underwater sound. Its adop- 
tion by the USRL has facilitated accurate calibration 
of standards for frequencies ranging from about 10 c 
to 2.5 me, with the possibility of attaining much 
higher frequencies. The lower frequency limit was 
determined by the difficulty of making low-frequency 
measurements in a shallow lake and not by failure 
of the reciprocity method. In the range from 100 c 
to 100 kc, this method is accurate to within ±1 db. 
A comparison of standard levels obtained by reciproc- 
ity calibration at USRL with those independently 
established at British laboratories showed excellent 
agreement. Other calibration methods (low fre- 
quency pressure tank built by BTL and the CMF 
self-calibrating condenser hydrophone, built by the 
Massachusetts Institute of Technology [MIT]) suc- 
cessfully extended the low frequency limit to below 
1 c. 

1. 1.2 Development of Standard 

Instruments 

Although the BTL, operating under an OSRD 
contract, was largely responsible for the development 
of hydrophones and projectors suitable for use as 
standards, many other laboratories constructed 
standards suited to the particular applications in 
which they were interested. Instruments, employing 
piezoelectric crystal, magnetostrictive, condenser and 
electrodynamic coupling, cover collectively the 1 c 


1 


2 


INTRODUCTION 


to 2.5 me frequency range. Many of these instru- 
ments have flat responses over wide frequency ranges. 
The 3A type crystal hydrophone, for example, has a 
flat response from about 100 c to 25 kc, and the XMX 
crystal hydrophone has a flat response within ±2 db 
from below 100 c to about 100 kc and hence were 
useful not only for single frequency measurements, 
but also for recording transients, and for measuring 
signals covering a wide frequency band. In the ma- 
jority of cases, these instruments were also character- 
ized by mechanical ruggedness and stability of 
calibration. Considering the fact that acoustic stand- 
ards for underwater applications were almost non- 
existent before 1940, the rapidity and comprehensive- 
ness of subsequent developments in the art are 
noteworthy. 

1.1.3 Objectives of Tests and 

Calibrations 

Although the necessity for development and pre- 
production tests of underwater sound gear was soon 
recognized, the specific objectives of such tests and 
calibrations became apparent with further develop- 
ment. Although this problem is still not completely 
solved, much has been learned concerning the signifi- 
cance of the various factors which characterize elfi- 
cient operation of sonar gear. Because of the large 
number of tests required and the limited facilities 
available, it was necessary to concentrate on the 
determination of these factors which depend upon 
the application for which the equipment was de- 
signed. While factors such as frequency response, 
directivity, impedance, signal-to-noise ratio, and 
efficiency were of significance in the majority of meas- 
urement programs, other quantities including rear 
response, side lobes, tuning, harmonic distortion, 
and variability with temperature and pressure often 
were of equal importance. 

Quantitative relationships between the foregoing 
quantities and the operational efficiency of under- 
water sound devices were only imperfectly under- 
stood at the initiation of the program. Theoretical 
and experimental studies which were undertaken 
provided a more exact understanding of these rela- 
tionships and a sounder basis for compromise be- 
tween competing factors in design, and, in turn, a 
basis for the selection of important parameters to be 
measured in development and calibration tests. 
Thus, the relationship which existed between devel- 


opment, calibration and testing, and operational and 
tactical application of the equipment, was of ines- 
timable value in unifying the program as a whole, 
and in directing it toward its prime objectives. 

Testing Technique 

With reference standards available, and with in- 
creased knowledge of significant parameters, the 
technique of actually performing a calibration test 
remained to be considered. Because these parameters 
were characteristic of the devices under test, and had 
to be distinguished from extraneous effects produced 
by the test location and by the equipment involved, 
a large part of the effort of the USRL, as well as that 
of other laboratories engaged in equipment develop- 
ment, was directed toward devising measurement 
techniques. The principal problems in testing gear 
in laboratory sites arise from the limited extent of 
the testing medium. Acoustic reflections from the 
surface, bottom, and shores of the body of water in 
which the tests are made and the limited testing dis- 
tance available required the development of means 
for eliminating the effect of these factors. To this 
end baffles and screens, directional sources, proper 
orientation of instruments, pulses and noise bands 
have been used for eliminating the effect of reflec- 
tions, and spherical wave corrections have been 
applied to compensate for effects caused by short 
testing distance. In addition, equipment was de- 
signed to reduce the time and labor necessary for 
performing tests with maximum use of available 
facilities. Among devices in this category are auto- 
matic recorder devices (linear and polar), special 
amplifiers, arrangements for rapid changes in driv- 
ing and receiving impedances, adjustable and inter- 
changeable rigging gear, and computing aids to 
facilitate the reduction of data. Techniques for test- 
ing in indoor tanks under varying conditions of tem- 
perature and pressure, for testing at high power 
inputs, and for measuring impedance under various 
conditions were developed as the need arose. With 
experience, the technique of testing underwater 
sound devices grew as an art as well as a science. 
Close liaison with developments in the field of under- 
water sound was essential to insure the adaptability 
of testing techniques and facilities. This flexibility 
made possible the realization of the potentialities of 
laboratory testing in the development of underwater 
sound devices. 


FUNDAMENTAL STUDIES AND OPERATIONAL PERFORMANCE 


3 


• 2 ROLE OF PRECISION ACOUSTIC 
MEASUREMENTS IN THE 
IMPROVEMENT OF UNDERWATER 
SOUND EQUIPMENT 

^ Improvements in Transducer Design 

An example of the importance of precision acous- 
tic measurements and fundamental research in im- 
proving sonar gear is afforded by developments in 
transducer design and construction in the period 
1940-1945. Two types of electroacoustic coupling, 
magnetostrictive and piezoelectric, had been found 
most useful. Fundamental investigations of magneto- 
strictive phenomena conducted by Division C, Navy, 
and industrial laboratories have greatly increased 
knowledge of the source of power losses in magneto- 
striction transducers. Application of this knowledge 
led to a 400 per cent increase in transducer efficiency. 
1 his increase in efficiency not only markedly im- 
proved operation but also greatly reduced the bulk, 
cost and critical materials requirements of the equip- 
ment. 

Improvement in equipment using piezoelectric 
crystal coupling closely parallels that obtained for 
magnetostrictive equipment. Thus, initial efficiencies 
of the order of 10 to 20 per cent have been increased 
to values as high as 75 to 80 per cent. 

Improvements in transducers have not been lim- 
ited to increased efficiency. The development of am- 
monium dihydrogen phosphate [ADP] crystals made 
possible crystal transducers with negligible tempera- 
ture dependence and, more important, with ability 
to withstand greater temperature extremes without 
damage. Furthermore, new construction methods 
for both crystal and magnetostriction transducers 
made possible the fabrication of transducers with 
special characteristics. Many other examples of spe- 
cific improvement are available to demonstrate the 
importance of fundamental research. 

*•2 2 Improvements in Dome Design 

In 1942, one of the most pressing sonar problems 
was that of designing domes which, when mounted 
on vessels, would not hopelessly distort the direc- 
tional patterns of the transducers they enclosed. In- 
ternal reflections from the walls of domes, then 
available, were nullifying the effort to attain satis- 
factory directional pjatterns. 7 his effect, together 


with the high transmission loss through the dome 
wall sharply reduced the effectiveness of the equip- 
ment. British experience in meeting this problem, 
coupled with an experimental research program on 
domes at the Na\ al Research Laboratory, at Division 
6 NDRC laboratories, and at the Bell Telephone 
Laboratories, supplemented by theoretical research, 
in which the Division was given invaluable assistance 
by the Applied Mathematics Panel, effectively dem- 
onstrated the principles necessary to reduce internal 
reflections to a negligible amount. On the basis of 
these principles, various manufacturers were able to 
produce acoustically satisfactory domes with high 
mechanical strength without a corresponding in- 
crease in thickness of the dome wall. The solution of 
the dome problem during 1943 was so effective that 
dome construction was never again a major problem. 

13 FUNDAMENTAL STUDIES AND 

OPERATIONAL PERFORMANCE OF 
UNDERWATER SOUND EQUIPMENT 

1.3.1 Relation to Calibration and Test 

Measurements 

It has previously been stated that the goal of anti- 
submarine research is improvement in the opera- 
tional performance of gear. The relation of funda- 
mental studies to operational performance is largely 
indirect and finds expression in their influence on 
development. One of the purposes of these studies 
has been to determine the relation between the 
parameters (power output, response, directivity, sig- 
nal-to-noise ratio, etc.) of underwater sound gear to 
its effectiveness in operation. This relationship is 
important, as it helps determine by laboratory meas- 
urements on the gear itself its probable operational 
effectiveness. Such a technicjue provides many ad- 
vantages, including: (1) efficiency of time and effort, 
(2) closer liaison with development work, (3) the 
possibility of making tests under controlled condi- 
tions, at any time, and (4) clearly defined conclusions 
from tests. 

2 Relation to Development of 
Equipment 

These studies reveal, also, important information 
bearing on the equipment design. In evaluating the 
relationship between gear j^arameters and opera- 


4 


INTRODUCTION 


tional performance, they indicate the relative im- 
portance of the parameters and thus establish the 
changes in design most effective for improving op- 
erational performance of the gear. As desirable im- 
provements are often inherently contradictory, these 
studies may be useful in indicating the most advan- 
tageous compromise, not only for gear in general, 
but in view of particular conditions of service, and 
also for particular types of equipment. 

The studies have been carried out on a more com- 
prehensive basis directed toward evaluating the rela- 
tive effectiveness of different classes of gear by infor- 
mation obtained in controlled surroundings. 

1.3.3 Relation to Manufacturing 

Requirements and Specifications 

There is still another important application of 
iCundamental calibration measurements on under- 


water sound gear, namely, as an aid to the establish- 
ment of manufacturing acceptance requirements. 
Whenever it is necessary to manufacture equipment 
in production quantities, it is often impossible, be- 
cause of considerations of time and facilities avail- 
able, to test completely each unit produced. It is 
therefore necessary to examine critically those fac- 
tors of prime importance in determining satisfactory 
operation of the equipment, and to establish limits 
of acceptable performance. These limits should be 
primarily based on careful studies of the gear under 
controlled conditions, keeping always in mind the 
influence of the various factors on operational per- 
formance. 

Thus calibration measurements under controlled 
conditions not only are an aid in determining speci- 
fications but also suggest methods for determining 
whether manufactured equipment is within the lim- 
its set by the specifications. 


Chapter 2 


OPERATION AND APPLICATION OF UNDERWATER 

SOUND DEVICES 

By Leslie L. Foldy 


C ALIBRATION work ill uiiderwater sound is con- 
cerned primarily with devices which convert elec- 
tric energy into acoustic energy (projectors) and, con- 
versely, those which con\'ert the energy of a sound 
field into electric energy (hydrophones). Generically, 
these devices are known as electroacoustic transducers. 
To pro\ide a suitable background for the detailed 
discussion of the problems involved in calibrating 
transducers, this chapter presents a brief discussion of 
(1) the physical principles underlying the action of 
transducers, and (2) their uses. 

2 1 PRINCIPLES OF OPERATION 
OF TRANSDUCERS 

As already noted, energy conversion is the funda- 
mental purpose of transducers.**^ Most transducers can 
convert energy in either direction, that is, they are re- 
versible. According to the nature of the physical proc- 
ess used in the energy conversion transducers may be 
classed under four general headings, namely electro- 
dynamic, electrostatic, piezoelectric, and magneto- 
striction. The principles involved in each are dis- 
cussed here in a qualitative manner. A more cpiantita- 
tive keatment is given in Chapter 3. 

The simplest example of an electrodynamic trans- 
ducer is a moving-ribbon instrument (Figure 1) 
which consists of a rectangular metallic strip or rib- 
bon suspended in a magnetic field. Wdien the instru- 
ment is in a sound field, there is generally a difference 
in pressure established between its front and back. 
Since the mass of the ribbon differs from that of the 
magnet, relative motion of the two results, inducing 
an electromotive force in the moving ribbon. Con- 
versely, if an alternating current flows through the 
suspended ribbon, the forces on it due to interaction 
of the current and the external magnetic field pro- 
duce vibration. This vibratory motion in turn creates 
a sound field in the meditnn. Since the process de- 

a These matters are discussed in detail in the Division 6 vol- 
umes on magnetostriction and crystal transducers. 


TO ELECTRICAL 



Figure 1. Electrodynamic transducer (pressure-gradient 
type). 

j)ends on the pressure difference between the front 
and back of the ribbon, such an instrument is called a 
pressure-gradient hydrophone. 

Electrodynamic transducers in which operation de- 
pends on the value of the pressure at a point in the 
medium, rather than on the gradient of the pressure, 
may also be considered. A simple example is a trans- 
ducer containing a diaphragm mounted in a water- 
tight housing in such a manner that only one of its 
faces is exposed to the water (see Figure 2), and with 
the pressure in the housing adjusted to compensate 
for the external hydrostatic pressure. A coil encircling 
a fixed magnet is rigidly fastened to the unexposed 



Figure 2. Electrodynamic transducer (pressure type). 


5 


6 


OPERATION AND APPLICATION OF SONAR DEVICES 


FIX^P^CONDENSER \ 

COHDENSEH^-'^^lkj^ 

(DIAPH ^G M ) 

TO ELECTRICAL TERMINALS-^ 

Figure 3. Electrostatic transducer. 

side of the diaphragm. When the instrument is placed 
in a sound field, the diaphragm and coil are set into 
vibration relative to the magnet. This vibration 
changes the flux linked by the coil, thus inducing an 
alternating emf in it. Conversely, the force due to the 
interaction of an alternating current through the coil 
and the field of the magnet gives rise to vibrations of 
the coil and diaphragm, generating a sound wave in 
the medium. 

Electrostatic, piezoelectric, and magnetostriction 
transducers are generally pressure rather than pres- 
sure-gradient instruments. Thus, a condenser-type 
transducer uses an electrostatic field to convert acous- 
tic into electric energy and vice versa. An example of 
the condenser-type instrument is shown in Figure 3. 
Here a constant potential is applied between the 
plates of the condenser. When the instrument is 
placed in a sound field, the movable plate is set into 
vibration with respect to the fixed plate. This vibra- 
tion changes the distance between the plates, and con- 
sequently the capacity, of the condenser. Since the 
voltage across the plates is inversely proportional to 
the capacity, an alternating voltage is generated. Con- 
versely, the application of an alternating electric po- 
tential to the plates changes the force between them, 
and consequently causes vibration of the movable 
plate. 

The other two types of reversible transducers de- 
pend on less familiar physical phenomena. The first 
of these is the piezoelectric effect. It has been found 
that certain crystals, when subjected to compression, 
exhibit electric charges on their faces; under tension 
the charges are reversed. The inverse piezoelectric ef- 
fect also exists: the crystals expand when a potential 
of one sign is applied across the faces and contract 
when an opposite potential is applied. 


TO ELECTRICAL TERMINALS 



Figure 4. Piezoelectric transducer. 

The faces on which the charges appear when the 
crystal is subjected to stress depend on its structural 
properties. A tourmaline crystal, for example, is one 
of the simplest insofar as piezoelectric effects are con- 
cerned. Tourmaline possesses a single piezoelectric 
axis such that a stress in the direction of this axis pro- 
duces charges on the faces normal to it. Thus, in Fig- 
ure 4, if a stress is applied in the direction of the piezo- 
electric axis Z, charges appear on the faces a and a'. 
Conversely, applying a potential between a and a' 
causes the crystal plate to expand or to contract in 
thickness (depending on the sign of the potential) 
in the Z direction.^ 

The simplest type of piezoelectric transducer, there- 
fore, comprises a tourmaline crystal in contact with 
two metal condenser plates. Application of an alter- 
nating potential to the plates causes the crystal al- 
ternately to expand and contract. Upon immersing 
the system in water, a sound field is generated by the 
vibration of the plates. Conversely, vibrations pro- 
duced by placing the crystal and condenser plates in a 
sound field give rise to an alternating voltage on the 
plates.*^ 

The fourth type of transducer, shown in Figure 5, 
operates on the principle of the magnetostrictive ef- 
fect, which bears certain similarities to the piezo- 
electric effect. If a rod or tube of ferromagnetic mate- 
rial (iron, cobalt, nickel, or various alloys containing 

b The changes in length involved are small. For tourmaline, 
the fractional change in length per unit electric field (1 volt per 
cm) is 1.93 X 10 — lo cm’^^ gm — sec. 

c It may be remarked that the amplitude of vibration for a 
given impressed alternating voltage and the magnitude of the 
induced alternating voltage for a given impressed sound field 
are both maximized when the frequency of the impressed volt- 
age, or sound field, coincides w'ith the natural mechanical reso- 
nance frequency of the crystal. 


DIRECTIVITY OF TRANSDUCERS 


7 


these metals) is brought into a magnetic field parallel 
to its length, its length is changed slightly.** This 
change of length is independent of the sign of the 
field and may be either an increase or decrease, de- 
pending on the nature of the material, its previous 
treatment, the degree to which it was previously mag- 
netized, and the temperature. This phenomenon is 
reversible: in other words, if a previously magnetized 
rod of nickel is stretched, the magnetization of the rod 
is decreased; if the same rod is compressed, the mag- 
netization is increased. 

If a coil of wire is now put around the rod, an emf 
is induced in it by the changes in the magnetic flux 
caused by the elastic deformation. In a similar man- 
ner, changes in the rod’s magnetization due to an ex- 
ternal alternatihg current induce periodic oscilla- 
tions in its length.® 

This simple magnetostriction transducer is, in ef- 
fect, a rod of ferromagnetic material surrounded by a 
coil. 

It should finally be pointed out that there are, in 
addition to the reversible transducers which have 
been considered, irreversible ones. In the carbon 
microphone, for example, changes in pressure on a 
diaphragm caused by an impinging sound field pro- 
duce changes in the electrical resistance of contacts 
between carbon particles and give rise to an alternat- 
ing current, provided that a source of constant poten- 
tial is present. 

22 DIRECTIVITY OF TRANSDUCERS 

The variation with direction of the emitted sound 
intensity, referred to the transducer acoustic axis,^ is 
called the directivity of the transducer on transmit- 
ting. In a similar manner, the directivity of the trans- 
ducer on receiving is defined as the variation of the 
output voltage for a plane-wave sound of given in- 

<i The fractional change in length is approximately ten parts 
in a million for fields up to 1,000 gauss. 

e It is to be noted that, since the increase or decrease in length 
of the magnetostrictive rod is independent of the sign of the 
magnetic field, an originally unmagnetized rod vibrates at twice 
the frequency of the impressed field, while a rod which has been 
sufficiently magnetized by another constant (polarizing) field 
vibrates at the impressed frequency. Maximum amplitude oc- 
curs when the impressed and natural frequencies of vibration 
coincide. 

^The transducer acoustic axis is arbitrarily selected, but is 
usually chosen to be an axis of symmetry of the instrument, such 
as the normal to a plane vibrating diaphragm. 



Figure 5. Magnetostriction transducer. 

tensity incident in various directions with respect to 
the axis. 

Directivity on transmitting results from the super- 
position at a point in space of sound wavelets emitted 
by different portions of the diaphragm, and conse- 
quently having different amplitude and phase. On re- 
ception the directivity is the result of the incidence 
on different portions of the transducer diaphragm of 
wavelets of sound which have, at the diaphragm, dif- 
ferent amplitudes and phases depending on the orien- 
tation of the diaphragm relative to their direction of 
propagation. 

In general, the directivity, both on transmitting 
and on receiving, is determined by the ratio of the 
sound wave length to the diaphragm dimensions.^ 
For a plane piston diaphragm with dimensions large 
compared to a wave length, the transmitted sound 
field is, at large distances from the transducer, cen- 
tered about the transducer acoustic axis, and the 
sound is emitted in the form of a “searchlight” beam. 
Conversely, on reception, the transducer is sensitive 
only to sound signals incident in directions close to 
and along the axis. Subsidiary maxima (side lobes) in 
directions far from the axis usually are present. These, 
however, are strongly dependent on the exact velocity 
distribution along the diaphragm, and are customar- 
ily small compared with the main or searchlight 
beam.** If the wave length is large compared to the 
diaphragm dimensions, the sound is emitted uni- 
formly in all directions. In this case, the transducer 
response on reception is independent of the direction 
of sound incidence. An intermediate case is a “line” 

g Transducers obeying the reciprocity theorem (see Chapters 
3, 4, and 5) have the same directivity on transmitting and on 
receiving. 

h By constructing a transducer with a plane diaphragm and 
w'ith a velocity distribution which is greatest at the center of the 
diaphragm and which decreases toward the edges (shading), it is 
po.ssible to obtain a directivity distribution with side lobes much 
smaller than in the case of a constant velocity distribution; the 
width of the main beam is, however, larger in this case. 


8 


OPERATION AND APPLICATION OF SONAR DEVICES 


transducer, with a pulsating cylindrical surface of a 
length which is large, and a diameter which is small, 
compared to a wave length. Here the sound is emitted 
uniformly in all planes containing a cross section of 
the cylinder, while in perpendicular planes the sound 
is concentrated in a beam. 

2 3 APPLICATION OF UNDERWATER 
SOUND DEVICES 

The applications of underwater sound devices in 
naval operations may be roughly divided into two 
categories: tactical and nontactical.^ Tactical applica- 
tions include the detection of surface vessels, subma- 
rines, torpedoes, mines, and underwater phenomena 
by surface vessels and submarines. The non tactical 
applications include: fathometer depth determina- 
tions by surface craft and submarines; monitoring of 
noise output by submarines; underwater communica- 
tion by code or voice; fundamental studies on sound 
propagation in the ocean, on ship noises, on under- 
water phenomena, etc., designed to aid in the tactical 
employment of sonar devices; and finally, the calibra- 
tion of sonar gear with standard projectors and hy- 
drophones. 

Tactical Applications 

Tactical applications of sonar gear generally in- 
volve either echo ranging or listening. In echo rang- 
ing, the devices emit either a pulse or a continuous- 
wave signal, which may be of either sonic or super- 
sonic frequency. If the signal strikes a target, part of 
its energy is reflected back to the emitting device (the 
projector) which receives the signal. On the basis of 
characteristics such as frequency shift and time delay 
between emission and reception, conclusions may be 
drawn regarding the range, bearing, speed, and na- 
ture of the target. In listening, any supersonic or sonic 
signal or noise emitted by a target actuates a receiving 
device which, from the characteristics of the signal or' 
noise (intensity, frequency, and direction), enables 
conclusions to be drawn regarding the range, speed, 
bearing, and nature of the target. 

Echo-Ranging Gear 

Conventional searchlight echo ranging gear as em- 


1 The distinction l)etvveen tactical and nontactical applica- 
tions is, of course, rather arbitrary and by no means rigid. 


ployed in the last war may be classified according to 
its tactical use, as antisubmarine or prosubmarine. 
Searchlight-type gear is in general use in both the 
United States and British Navies. Searchlight-type 
echo-ranging gear consists of a projector with either 
a square or a circular diaphragm operating on the 
piezoelectric or on the magnetostrictive principle. 
The dimensions of the projector are, as a rule, con- 
siderably larger than the wave length of sound 
emitted, so that a relatively narrow beam is formed. 
The axis of this beam may be rotated by training the 
projector in a horizontal and sometimes in a vertical 
plane. As a result of the application of a suitable pulse 
voltage, the projector emits supersonic pulses of from 
10 to 200 milliseconds duration. 

Immediately after transmission, the projector is 
switched from the transmitter to the receiver. Any re- 
ceived reflected pulse of supersonic frequency is 
either directly rectified and presented visually on the 
screen of a cathode-ray tube or, more customarily, is 
heterodyned to an audible frequency and presented 
through a loud-speaker. The time delay between 
transmission and reception is a measure of the range 
of the target from the projector; the orientation of 
the projector (or beam) axis in space at the instant of 
transmission gives the relative bearing of the target; 
any difference between the frequency of the emitted 
and the received pulse (the Doppler effect) is a meas- 
ure of the speed of the target; and the quality of the 
received (heterodyned) pulse often throws informa- 
tion upon the nature of the target (for example, dis- 
tinguishes a submarine from its wake). Thus, con- 
siderable information about the position, motion, 
and nature of the target is obtained. 

By systematically training the projector in azimuth 
(i.e., by following a definite search plan) it is possible 
to sweep a sector of ocean where the presence of the 
target is either known or suspected. In echo ranging 
by surface craft in search of submarines, the target 
may be found at different depths and ranges so that 
the width of the beam of the main projector is not 
always adequate for maintenance of contact. In this 
case, auxiliary projectors may be employed. 

A list of representative tactical applications of vari- 
ous searchlight echo-ranging devices follows: 

1 . Detection of submarines by surface craft. 

2. Detection of submarines by harbor installations. 

3. Detection of submarines and surface craft by 
submarines. 

4. Detection of small objects such as mines, tor- 


APPLICATION OF UNDERWATER SOUND DEVICES 


9 


pecloes, landing obstacles, and shoals by surface 
craft, submarines, and swimmers. 

Listening Gear 

The second tactical application of sonar gear in- 
volves listening, in which any supersonic or sonic 
signal or noise emitted by the target and reaching the 
hydrophonej is picked np and presented to the opera- 
tor. If the incident signal is not of an audible fre- 
cpiency to begin with, the resultant hydrophone ont- 
pnt may be heterodyned to an audible frequency. 
Intensity, frequency, and direction of incidence of 
the signal enable conclusions to be drawiv regarding 
the nature, speed, range, and bearing of the target. In 
particular, if a directional hydrophone‘s is used, the 
bearing of the target may be determined from the 
direction of orientation of the hydrophone axis at the 
position of maximum response. 

The following specific tactical applications of lis- 
tening may be listed: 

1. Sonic and supersonic listening for submarines 
from antisubmarine craft, harbor installations, other 
submarines, and expendable devices, sono buoys, etc.‘ 
Supersonic listening is perhaps preferable to sonic 
listening in these cases because of the high directivity 
generally obtainable with supersonic gear.'“ Super- 
sonic hydrophones on antisubmarine craft, harbor 
installations, and submarines can also pick up the 
emitted echo-ranging pings of enemy submarines. 

2. Sonic and supersonic listening for surface craft 
from submarines. Sonic and supersonic listening, 
usually with more or less directional hydrophones, is 
widely used by submarines to detect the noise output 
of merchant craft in convoys, of antisubmarine craft, 
and of other enemy warships. Submarines also often 
overhear on supersonic listening gear the echo-rang- 
ing pings of antisubmarine craft. 

3. Supersonic listening for torpedoes from surface 


j Hydrophones used in practice are of a variety of construc- 
tions, sizes, and shapes, but usually operate on the piezoelectric 
or magnetostrictive principle. 

k The directional receiving hydrophone may be the )>rojector 
of the supersonic echo-ranging gear, hooked up electrically for 
signal reception. 

1 A sono buoy is a device containing a hydrophone and a 
radio transmitter. When the hydrophone receives a signal from 
a target (the submarine), it is transmitted by means of a radio 
link to patrolling antisubmarine air or surface craft. 

m The high directivity is desirable for two reasons: it leads to 
a greater hearing accuracy and minimizes the self and ambient 
noise pickup of the gear. 


craft and submarines. Cavitation noise from a moving 
torpedo may be detected by directional supersonic 
listening gear, and appropriate evasive action may be 
taken. 

2.3.2 Nontactical Applications 

Nontactical applications of sonar gear include the 
use of standard projectors and hydrophones for the 
calibration of other kinds of sonic and supersonic de- 
vices," the determination of physical parameters of 
sonar devices, and analysis of the physical parameters 
of a particular type of gear to determine its suitability 
for the tactical or other purpose at hand. A somewhat 
related nontactical application is the use of standard 
projectors and hydrophones for testing echo-ranging 
and listening gear installed on antisubmarine craft 
and on submarines, and for permitting a submarine 
to monitor its own sonic and supersonic output. 

Other nontactical applications of sonar gear in- 
clude fathometer depth determination by surface 
craft and by submarines. Fathometer gear, similar in 
construction and operation to supersonic searchlight 
echo-ranging gear, emits short supersonic pulses, and 
then receives and mechanically records reflections 
from the ocean bottom. Sonar gear (standard hydro- 
phones and projectors) may also be used to study the 
sound output of disturbances as well as to determine 
the sound-absorbing and reflecting properties of vari- 
ous materials. A further nontactical application of 
transmitting projectors and receiving hydrophones 
involves the use of code or speech-modulated super- 
sonic signals for underwater communication between 
surface and sul)surface craft. 

Finally, an important nontactical application of 
sonar gear involves its use in fundamental studies 
of sound propagation in the sea under various oceano- 
graphic, surface, and bottom conditions, with the at- 
tendant study of surface and bottom reflection, refrac- 
tion, attenuation, scattering, and reverberation. The 
study of various types of noise background (ship’s 
noise, ambient noise, and target noise) and of the 
reflecting power of various targets should be men- 
tioned in this connection. Such fundamental studies 
are useful in determining the relation of gear opera- 
tional efficacy to gear parameters with a view toward 
optimum sonar design. 

» Calibration usually involves the detenniuatiou of the axis 
response, directivity, efficiency, anti power output of the device. 
(See Chapter 4.) 


Chapter 3 

GENERALIZED THEORY OF ELECTROACOUSTIC 

TRANSDUCERS 


By Leslie L. Foldy and Henry Primakoff 


31 INTRODUCTION 

F ollowing the qualitative discussion of various 
simple idealized transducers given at the begin- 
ning of the preceding chapter, a generalized theory of 
linear passive electroacoustic transducers is now de- 
veloped. Attention is centered on deriving relation- 
ships true for all linear passive transducers, rather 
than on a detailed analysis of particular types. There 
is a rather complete analogy between the theory of 
electroacoustic transducers, taking into account the 
properties of the sound field, and that of electro- 
mechanical transducers,^^ where only one mechani- 
cal degree of freedom is present. Consequently, this 
discussion is carried through in fairly abstract terms, 
insofar as this can be done without undue mathemati- 
cal complexity. Analogies are pointed out as they 
occur. 

An electroacoustic transducer is a device for trans- 
forming electric energy into acoustic energy, or vice 
versa. If all the energy delivered by the transducer to 
the electric or acoustic systems to which it is con- 
nected is derived from power absorbed by the trans- 
ducer from these systems, the transducer is said to be 
passive. This does not prohibit the presence of active 
internal sources of power such as are used to provide 
polarizing voltages and currents in some types of 
transducers, provided that these internal sources do 
not supply power to the electric or acoustic systems 
to which the transducer is connected. 

Schematically, an electroacoustic transducer may 
be represented as a pair of electric terminals, by 
means of which connection to electric systems is 
made, and a closed surface‘s which is in contact with 
a medium capable of propagating sound. The sim- 
pler electromechanical transducer, which may be 
represented schematically by a box with a pair of 
electric terminals and a pair of mechanical terminals, 
is shown in Figure 1. 

Only those transducers are considered in which the 
acoustically active part of the surface, the diaphragm, 

a Only part of this surface need be acoustically active. 


Vn 



Figure 1. Electroacoustic transducer represented as a 
four-terminal electromechanical network. 

vibrates in such a manner that its normal velocity is 
the same at all points (rigid vibration). It is possible 
to remove this restriction and develop the theory for 
any type of vibration, as discussed briefly later. The 
former case, however, is considerably simpler mathe- 
matically and the treatment of it given here contains 
the essential physical principles of the problem. 

The quantities of interest, expressed as functions 
of time t, are the following: the voltage E( t ) across the 
electric terminals of the transducer, the current I(t) 
into the electric terminals, the normal velocity Vn(t) 
of the diaphragm, and the total force F(t) on the dia- 
phragm. This total force may be considered as the 
integral over the diaphragm of the pressure at each 
point on its surface. The pressure and particle veloc- 
ity of the sound field at any point in the medium are 
also of interest. 

Consider only the steady state, where all quantities 
vary harmonically with the time with the same fre- 
quency: E{t) = E ej^*, Vn{t) = Vn etc. For electro- 
acoustic transducers in which the diaphragm vibrates 
rigidly, it is found that any two of the four quantities 
E, /, F, and determine the values of the other two. 
Take I and as independent variables. The most 
general equations for a linear transducer of this type 
are then 


F = ZoVn-\- k I 

(1) 

E = k'Vn + I, 

(2) 


10 



IMPEDANCES 


11 


where Zq, k, k', and Zj, are independent of E, I, F, and 
Vn but are in general functions of frequency.*’ Equa- 
tions of the type of (1) and (2) have been shown to 
apply to the majority of electroacoustic transducers 
now in use, at least for limited ranges of the variables. 

Considering the physical significance of the terms 
in equations (1) and (2), it is seen that Zq is the force 
on the diaphragm divided by the velocity when the 
current is zero. Let Zq be the open-circuit mechanical 
impedance of the transducer. The constant k gives 
the force developed per unit current into the trans- 
ducer when the diaphragm is completely constrained 
(^Vn = 0). Therefore k is called the electroacoustic 
transfer constant. Similarly, k', the open-circuit volt- 
age per unit diaphragm velocity, is called the acous- 
toelectric transfer constant. The term Zj, gives the 
voltage developed for unit current when the dia- 
phragm is constrained from moving and so is called 
the blocked electric impedance. 

The equations for a simple electromechanical sys- 
tem are of precisely the same form as equations (1) 
and (2). This is to be expected, since the assumption 
that Vn is constant reduces the problem to one me- 
chanical degree of freedom. The acoustic case, how- 
ever, includes the properties of the sound field, as 
discussed below. 


transducer converts acoustic energy into electric en- 
ergy, we may write 

F = F^-ZrVn. (5) 

The interpretation is somewhat more complicated 
in this case: The term Fq really consists of two parts, 
^0 = ^inc + frigid diffr* The first, E'inc, IS the force on 
diaphragm that would be present if the transducer 
had no effect on the incident sound field. The second 
part, /^rigid diffr > be considered as representing the 

force on the diaphragm due to the sound that would 
be diffracted by the transducer if the latter were per- 
fectly rigid. The symbol z,. represents the radiation 
impedance, and the term (— z,. v^) is the force on the 
diaphragm due to the additional sound pressure 
created by the latter’s motion in the sound field. 

Finally, when the transducer is used to convert 
electric energy into acoustic energy, there is no ex- 
ternal sound field and the equation becomes 

F = - z, (6) 

It should be noted that equation (5) corresponds 
to the equation for an electromechanical transducer 
coupled to a source of generated force Fq and of in- 
ternal mechanical impedance z,.. 


3 2 COUPLING CONDITIONS 

Consider now the relationships which obtain when 
an electroacoustic transducer is coupled to electric 
elements or to a medium capable of propagating 
sound. When the transducer is used to convert acous- 
tic energy into electric energy, it is terminated in an 
electric impedance that is, there is a load imped- 
ance Z 2 , across its electric terminals. In that case at all 
times 

E=-Z^L (3) 

When the transducer is used to convert electric 
energy into acoustic energy, a source of voltage £0 
of internal impedance Zjnt is connected to the electric 
terminals. Then 

£ = £„-Zi„./. (4) 

The problem of coupling on the acoustic side may 
be treated formally in a similar manner. When the 

b The linearity of the equations insures the possibility of 
treating functions with any time dependence by superposition 
using Fourier analysis. 


33 IMPEDANCES 

The next problem is the determination of the effec- 
tive impedances, electric and acoustic, of the trans- 
ducer under various conditions of coupling. Begin 
with the effective electric impedance, defined as the 
ratio of voltage to current Ejl. The value of this im- 
pedance depends on the nature of the acoustic cou- 
pling. Consider the case of the transducer in an 
infinite source-free medium. Then the force on the 
diaphragm is given in terms of the normal velocity 
Vn on the surface by equation (6) as £ = — z^. 

Substituting (6) in equations (1) and (2), we find 

- Zr "n = Zo Kn + kl (7) 

and 

E = k' v^ + Z^ I. (8) 


Solution of these equations shows that the effective 
electric impedance Z^^ is given by 


_ £ _ 2 _ kk' 

I ^ Zq + Z^ 


( 9 ) 


12 


GENERALIZED THEORY OF ELECTROACOUSTIC TRANSDUCERS 


I’he difference between Z^i and is called the mo- 
tional impedance Z,„. Thus 


Z,,i Zf>i Zk 


kk' 

Zq + Zr 


(10) 


Z„j being the contribution to Z^i which results from 
the motion of the diaphragm. 

Consider now the effective acoustic impedance 
defined as the ratio of force to normal velocity, F/ v^. 
The term depends on the electric coupling condi- 
tions. Suppose a load impedance Z^ is connected to 
the electric terminals so that, from equation (3), 
E = — Substituting (3) in the basic equations 
(1) and (2), we obtain 


F = z^v, + kl ( 11 ) 

and 

-Z^I = Fv, + Z,I. (12) 


Solution of these equations shows that 


z 


ac 


F 


^0 ~ 


kk' 


(13) 


3.4 SENSITIVITIES 


of the sensitivity S(R) depends upon the properties of 
both the transducer and the medium in which it is 
operating. 

ft is now necessary to express the pressure p(R) at 
any point R in the medium in terms of the normal 
velocity u,, of the diaphragm. This relationship can be 
shown to be 


p(R) = f 0(R,r) dv = 

(14) 

Here the integral is taken over the acoustically ac- 
tive portion of the transducer surface, the diaphragm 
S^; p is the density of the medium; o) is the angular 
frequency of the sound wave (27r times the frequency); 
and G(R, r) is the so-called Green’s function.^ Physi- 
cally, it may be defined as the pressure which would 
be produced at the point R as a result of a point 
source of unit strength placed at the point R', if the 
diaphragm of the transducer did not move. G(R, r) is 
simply G(R, R') for R' taken at the point r on the 
closed transducer surface S. ft can be shown that such 
a function can in principle be calculated for all ordi- 
nary surfaces S, and that it is symmetric in its argu- 
ments, that is, G(R, r) = G(r, R). The function g(R) 
is introduced simply as an abbreviation for the 
integral 


The various impedances associated with an electro- 
acoustic transducer have been expressed in terms of 
the fundamental transducer constants and the condi- 
tions of coupling. The sensitivities of a transducer are 
now considered.^^ Here, two types of sensitivity are of 
interest: the transmitting or electroacoustic sensitiv- 
ity and the receiving or acoustoelectric sensitivity. Ex- 
pressions for these are obtained and a proof of the 
reciprocity theorem, which is basic in much of under- 
water sound calibration work, is given. 

The transmitting sensitivity .S(R) of a transducer is 
defined as the ratio of the pressure developed by the 
transducer at the point R, when driven electrically, to 
the input current to the transducer.'^ fn practice, the 
|)oint R is taken as a point at unit distance (1 meter) 
on the axis of symmetry of the transducer. The value 

c The transmitting and receiving sensitivities defined liere 
l)ear a close relationshi)> to the transmitting and receiving re- 
sponses definetl below. (See Chaj)ter 4.) 

<1 For simplicity we shall denote a point in the medium by its 
position vector R from an arbitrary origin rather than by its 
coordinates. 


g(R) = f r;(R, r) dr. 
Js, 


(15) 


Since we are considering the transmitting sensitiv- 


e Green’s function, G(R, R'), is defined mathematically as a 
solution of the wave equation, which has a pole of residue unity 
at the point R = R', which satisfies the boundary condition 
9G (R, R')/9n m 0 on the closed transducer surface S, and 
which, as |R — R'| oo, behaves like 



-xiR-R' 


|R - R | 


(\ =r wave length; ^ function whose nature is de- 

termined by the surface S), that is, it resembles an outward 
travelling wave. Green’s function can be shown to exist mathe- 
matically for all ordinary surfaces S. It can also be shown that 
in the particidar case of a piston-like diaphragm in an infinite 


2e 




rigid baffle, G(R, R') | — R'l 

surface of the piston or baflle. 


,when R' = r lies on the 


SENSITIVITIES 


13 


ity, equation (6) applies and F = — z,. v^- Solving for 
v„ from equations (6) and (1), and substituting the re- 
sult into equation (14), the transmitting sensitivity is 
obtained as 

= <'«i 

Thus the transmitting sensitivity depends on the fre- 
quency / = <i)/27r, the density of the medium p, the 
open-circuit mechanical impedance Zq, the electro- 
acoustic coupling constant k, and the radiation impe- 
dance z,., as well as the integral of Green’s function 
over the surface of the transducer. 

Consider next the receiving sensitivity M. Suppose 
that in the abselice of the transducer from the me- 
dium there is present a sound field pi,jc(R), whose 
value at the position of the acoustic center of the 
transducer Rq (when the latter is not present in the 
medium) is pi„c(Ro).^ Then the ratio of the open- 
circuit voltage generated by the transducer when in 
the medium to pinp(R„) is dehned as the receiving 
sensitivity M.^ 

The value of M depends upon the type of wave, 
that is, pine. For practical applications, the important 
case is that for which is a spherical wave with cen- 
ter at some point R^. The plane wave sensitivity can 
be considered to be the sensitivity to spherical waves 
when R,. is inhnitely distant from R„. In this case pi„e 
can be written as 


fao(«-) I ■■ (17) 

The actual pressure present at a point R when the 
transducer is in the medium must now be found. If 
the transducer diaphragm did not move, the pressure, 
from the definition of Green’s function, would be 


is present in the medium but the transducer surface 
has a velocity v^, the pressure at a point R in the 
medium is as given by equation (14). If these two pres- 
sures are added, the actual pressure at R, p(R), when 
pine is present, is obtained and the transducer dia- 
phragm has a velocity Vn. Thus 


p(R) = ^G(K, R,) 


G(R,r)dr (18) 


= 4-G(R, R„)-^r;„g(R). 

Since the total force on the transducer diaphragm is 
the integral over the diaphragm of the pressure at 
each point on its surface, then 

f p(r)dr = <pf G(r, R,) dr - f g(r) dr. 

(19) 

Since G(r, R^.) = G(Rp, r) from the symmetry of 
Green’s function, the first term can be written as 
4>g(Rp). If we compare equation (19) with equation 
(5), we see that 

F, = 4^g(R,) (20) 

and 


z,. = 


]oyp 


g{r) dr = 


477 


G(r,r') dr' dr. (21) 


These quantities may be computed when Green’s 
function for the surface S is known. 

If (5) is substituted for F, with Fq and z^ given by 
equations (20) and (21), in the fundamental equa- 
tions (1) and (2) and the case where the transducer is 


4>G(R, Rp). 

On the other hand, if no incident sound pressure 


f The acoustic center is the center of symmetry if it exists, hut 
for this discussion it may he any arbitrarily chosen point on the 
transducer. The incident sound field pressure satisfies the 


wave equation 



(R) = 0, where \ is the 


wave length and y7“ b the Laplacian operator. 

s In practice the receiving response is always given for a uni- 
form plane wave normally incident on the transducer. 


hit can further be shown that, for an arbitrary incident 
sound field one has 

I f dp. (r) 

/,(R)=p,„^,R)-_P_t|^G(R,r)rfr 

The first two terms reduce to ^ G(R, R^) as shown in equation 
(18), if is the spherical wave of equation (17). If p(R)*is in- 
tegrated over the surface S^, the integral of the first term is 
what was called, while the integral of the second term is 
what was called. The last term is again — 


14 


GENERALIZED THEORY OF ELECTROACOUSTIC TRANSDUCERS 


electrically terminated in a load impedance Zj^ is con- 
sidered, so that E = — Zl I, Vn can be eliminated be- 
tween the equations and the following is obtained for 
the voltage output of the transducer: 


E ^ - ZjJ 


Zr 




^L + Z,- 


kk' Zo + Z; 




^0 + 


( 22 ) 


We are interested in the open-circuit voltage This 
is obtained from equation (22) by letting Z^— > oo; 
thus 


due to a point source of spherical sound waves^ at R, 
at a distance d from the acoustic center of the trans- 
ducer to the transmitting sensitivity S, measured at 
the point R, is a constant independent of all particu- 
lar characteristics of the transducer. The value of the 
constant is: \M/S\ = ^dXjpc, where X and c are the 
wave length and velocity of sound, respectively, and 
p is the density of the medium. The applications of 
this theorem are discussed in Chapters 4 to 7. 

The theorem is readily proved by taking the ratio 
of M as given by equation (25) with R^ = R, and S as 
given by equation (16): 


F = 


k'Fp 
Zq + Zf. 


(23) 


Since 


(Ro) = ^' 




|Ro-Rc 


M 

T 


k'g{K)djY^ 


Zq -f- Zf 


jmp ■ Zo + z/ 




(24) Taking the absolute value of this equation, and re- 
membering that |/t'| = |/j|, we obtain 


the receiving sensitivity, using equation (20) for Fq, is 


M 


Pinc(Ro) 


Fg(R,) |Ro-R,| 

-;|^1R,-RJ 

(Zq + Zr) e 


(25) 


3 5 PROOF OF RECIPROCITY THEOREM 


M _ 47rd _ 2dx 

S pa> pC 


(28) 


The reciprocity theorem, proved here for the case 
of a rigidly vibrating diaphragm, can be shown to 
hold for any general mode of diaphragm vibration 
where r'„(r) is a function of position on the dia- 
phragm. 


The reciprocity theorem will now be proved. This 
theorem applies to all transducers which obey the 
condition 

\k\ = \E\, (26) 

that is, equality of the absolute values of the electro- 
acoustic and acoustoelectric coupling constants. It is 
simple to show that this condition is satished for the 
various idealized transducers* considered in Chapter 
2. The statement of the theorem is as follows: The ab- 
solute value of the ratio of the receiving sensitivity M 


36 EFFICIENCIES 

Having discussed the sensitivities of a transducer 
and their relationship through the reciprocity theor- 
em, a treatment of efficiencies follows, beginning 
with the efficiency of the transducer on transmission, 
the projector efficiency. This is defined as the ratio 
of the total acoustic power output of the transducer 
to the electric power input. Several expressions for 
the projector efficiency Ep will be derived which will 
be useful for different purposes. 

The acoustic power output may be shown to be^^’^^ 


i Thus, for the case of the electrodynamic moving ribbon 
pressure-gradient transducer (see Chapter 2) k = — Bl, k' = Bl, 
where B is the magnetic flux density in the region where the rib- 
bon moves, and / is the effective length of the ribbon. Values 
of k and k' for other types of transducers are given in the 

literature.80, 8i 

J This restriction is not necessary. It may be shown that the 
reciprocity theorem is valid for any source distribution for the 
incident waves just so long as the distance between sources and 
transducer is large compared to the dimensions of either of 
them. 


2 

2 


p(R) t<„*(R) dz + 




pm* 



(29) 


k Another extension of the theorem is to the case of a series of 
transducers individually obeying reciprocity and coupled by 
electric and mechanical transformers. Then the condition \k'\ = 
|/i| will hold for the coupling between the input E, I and output 
F.v , ii it holds for the individual transducers, and the reci- 
procity theorem will be valid for the series considered as a unit. 


EFFICIENCIES 


15 


the asterisk denoting the complex conjugate and X 
being any closed surface containing the transducer. 
The electric power input is given by |/pRei where |/| 
is the absolute value of the (complex) current and Rpi 
is the real part of the effective electric impedance of 
the transducer Z^i. 

If X is chosen as a large sphere of radius d centered 
at the transducer, we have Vn = p/pc on Then 

the expression for the acoustic power output be- 
comes: 

i/j'’!'"" 

dX being now an element of area on the sphere and I 
the sound intensity. Introducing the directivity fac- 
tor 5, defined as-* 



with paxis the pressure at distance d on the axis of 
symmetry of the transducer, or, generally, on any 
fixed axis, one obtains for the projector efficiency 


g|M\xis 

pc 

\I\^ Rel 


(31) 

pc Re, A2 4Re, 


where S and M are the transmitting and receiving 
sensitivities; see equations (16), (25), and (28), 
Another possible choice for X is the surface S of 
the transducer. Then, is the normal velocity of the 
transducer surface, assumed constant over the dia- 
phragm. Thus from equations (6) and (29) we have 
the projector efficiency expressed as 


effective electric impedance, Z^i. It may be shown^® 
that a sufficient condition for Ep to be 100 per cent 
is Rj, = ro = 0, that is, the blocked electric and open- 
circuit mechanical impedances have no real parts. 

It is worth pointing out in connection with equa- 
tion (33) that the projector efficiency (at resonance) 
of a resonant transducer may be determined by 
purely electrical methods. (See reference 39.) In a 
resonant transducer, the response as a function of 
frequency has a sharp maximum when the impressed 
frequency coincides with a natural frequency of the 
transducer itself. If one measures the electric im- 
pedance of the transducer at a frequency well above 
and below its resonant frequency, the result will be 
essentially Zj,, the blocked impedance.™ Suppose now 
one measures the electric impedance of the trans- 
ducer at resonance in air. Then Zj. is approximately 
zero and the motional impedance (the difference be- 
tween the measured electric impedance and Z^,) will 
be —kk'/zQ. Next one measures the impedance at re- 
sonance in water. Then the motional impedance in 
water —kk'/{zQ + z^) is known. These three measure- 
ments suffice for the determination of Ep, if the trans- 
ducer obeys the reciprocity condition |^| = \k'\, so 
that |^|2 = |M'|. 

This may be seen as follows: At resonance the 
imaginary part of Zq + vanishes,^ and \zq Zr\^ = 
(^0 + ^r)“- Then equation (33) may be written as 

r \kE\ n 

K = 1^ P _ ^ei(^o T r^) ^ ro T r,. 

" Re, (ro + r,)2 \kE\ ~ 1W ’ 

To- 

rn 



(32) 


where is the real part of the radiation impedance z^. 

Finally, using equations (7) and (8) to find v^jl and 
equation (9) from Rg, and substituting the results into 
equation (32), we obtain 

E — 1^1^ /33\ 

' Re(z.— + 

V Zo + ZrJ 

( kk' \ 

Zft j — ] is the real part of the 

Zq Zfj/ 


1 For further discussion of the directivity factor 5 and of the 
directivity index = 10 log 5, see Chapter 4. 


where the second form uses the reciprocity condition: 

\k\^ = \kk'\. 

It is now seen that R^,, ^ , and IMi are the 

. . . , , . ^0 + ^0 
only quantities needed for a knowledge of Ep at re- 
sonance. All of these can be found by the method 
described, assuming that Vq is a slowly varying func- 
tion of frequency so that Tq, at the resonance fre- 
quency in air, is close in value to at the resonance 
frequency in water. 


m is in general a function of frequency but does not show 
resonance properties. Hence its value at the resonant frequency 
of the transducer may be found by joining the portions of the 
curve found above and below resonance by a smooth curve. 

n When the imaginary part of -f vanishes, the responses 
S and M have their maximum values; see equations (16) and (25). 


16 


GENERALIZED THEORY OF ELECTROACOUSTIC TRANSDUCERS 


37 MAXIMUM ELECTRIC POWER 
OUTPUT ON RECEPTION AND 
THRESHOLD PRESSURE 


3 8 GENERALIZATION OF THEORY TO 
ANY TYPE OF DIAPHRAGM 
MOTION 


Maximum electric power is transferred to a 
receiver when the electric load impedance of the 
receiver is the complex conjugate of the effective 
electric impedance of the transducer. Under this con- 
dition, by Thevenin’s theorem 


Rei 4Rei 


(35) 


The treatment of the theory of electroacoustic 
transducers given above for transducers in which the 
diaphragm velocity is the same at all points can 
readily be generalized to the case where there is no 
such restriction on the velocity distribution. The 
form that the generalization takes follows in outline. 
The fundamental equations for the transducer can 
be written as 


4R,i pc 47r& ”■ 

where Eg is the signal voltage across the load in the 
matched circuit, Eg^^c) is the signal voltage that would 
be developed by the transducer on open circuit, M 
is the receiving sensitivity, and Ej, is the projector 
efficiency. The last form of equation (35) follows 
from the last form of equation (31). Equation (35) 

suggests that the quantity -A?_, which has the dimen- 
47r8 

sions of an area and is usually called the effective 
area, has the significance of being the maximum 
cross section for energy absorption by the transducer 
from the sound field. This follows from the fact that 

l£-4B£Jis the incident intensity of sound, E^ never ex- 

ceeds unity, and maximum power is absorbed in a 
matched circuit. 

An important parameter of the transducer is its 
threshold pressure pf This is defined as the pressure 
in a uniform, plane-wave, free sound field propagated 
parallel to the acoustic axis of the transducer, which 
produces a signal power output in the load equal to 
the inherent thermal noise power in the load. (See 
Chapter 4 for a full discussion.) The noise power is 
taken in a 1 -cycle band and the transducer is sup- 
posed to be in a matched circuit. The noise power in 
the load in a matched circuit is one-half of the open- 
circuit noise power in the transducer (since noise 
pressures add in random phase). This open-circuit 
noise power in a 1 -cycle band is given by 4KT where 
K is Boltzmann’s constant and T is the absolute tem- 
perature of the device.”^® Consequently, the threshold 
pressure is given by the relation 

F b4KT^ 


p(r) = J z„(r,r') dr' + k(r) I (37) 

and 

E = j^k’(r')v„(r')dr’ + Z,I. (38) 

Here r and f are points on the transducer surface S; 
P(y) and r'„(r) are the pressure and normal velocity at 
the point r, respectively. The functions Zo(r,r'), /i(r), 
and A'(r') are functions characteristic of the trans- 
ducer which are the generalizations of Zq, k, and k in 
the simpler treatment given earlier. For coupling to 
an electric source of generated voltage E^ and inter- 
nal impedance Zjn^, the equation 

£ = £„ - Z,„, 7 (4) 

again holds. However, for coupling on the acoustic 
side we now have 

P(R) = Po(R) - ( O v,{t) dr' (39) 

7s 

where 

Po{^) = flno(K-) - 

(40) 

and 

and R is any point in the medium. (R may be taken 
equal to r.) Here G(r,r') is the same Green’s function 
introduced earlier. The quantity z,.(ur') is an acoustic 
radiation impedance continuous matrix which is the 


GENERALIZATION TO ANY TYPE OF DIAPHRAGM MOTION 


17 


generalization of the previously used radiation im- 
pedance z,.. When no sources other than the trans- 
ducer are present in the mediinn, the pressure at any 
point R in the medium is, by equations (39) and (40), 
given as 

p(R) = O M^') (42) 

I'he above equations allow the behavior of the trans- 
ducer to be calculated under any conditions. The 
various impedances, sensitivities, and efficiencies can 


be found in a manner analogous to that used above. 
1 hese calculations involve, in general, the solution 
of linear integral equations which can be solved in 
principle, though practical solutions may be difficult 
to obtain except in simple cases. The reciprocity 
theorem, equation (28), can be proved in this general 
case provided that 


|/,(r)| = \k'(r)\ 

(43) 

o 

II 

c 

(44) 


Chapter 4 

TYPES OF ACOUSTIC MEASUREMENTS 

By Eginhard Dietze 


T he choice of what should be measured is prob- 
ably as important a part of a testing program as 
any and requires a clear understanding of the nature 
and purposes of the tests and of the character and ap- 
plications of the device under test. Furthermore, the 
conditions under which the tests are made must be 
carefully controlled. The existence of controlled con- 
ditions is one of the principal reasons for substituting 
laboratory tests for field tests. 

The tester, furthermore, in order to carry out his 
task intelligently, must possess a broad knowledge of 
the applications of the device as well as of measure- 
ment technique. Assume, for instance, that an echo- 
ranging projector is to be tested. To set up a program 
for such tests, it is necessary to know what factors are 
important in echo ranging and how these factors de- 
pend on the physical characteristics of the device. 
Only then can tests be made that will throw light on 
how the device will perform in service and how its 
performance could be improved. 

A great deal of thought has been given by the 
Underwater Sound Reference Laboratories [USRL] 
to these questions. Based on these studies, the perti- 
nent physical characteristics that should be measured 
in a calibration test on an echo-ranging projector are 
(1) directivity (directivity index, horizontal and verti- 
cal beam widths, magnitude of largest side lobes), (2) 
frequency-response characteristic, (3) power output 
(efficiency), (4) selectivity, (5) threshold pressure, (6) 
receiving response, and (7) impedance. 

It is necessary to devise proper tests for the precise 
measurement of these characteristics. In acoustic tests, 
this is not always simple, and even with the greatest 
care it is usually not possible to equal the precision of 
electric circuit tests. All factors should be of the opti- 
mum design in order to achieve even a reasonable de- 
gree of precision. A first requirement in this connec- 
tion is one of testing equipment. Any effort expended 
in obtaining the best possible laboratory equipment 
will be well repaid. It is almost axiomatic that with- 
out such equipment the situation is hopeless. Assum- 
ing that such equipment is available, there are certain 
fundamental rules of testing which must be observed. 


The most important one, although very simple, is fre- 
quently violated, often with disastrous results. This 
rule is as follows: In a test, as in any experiment, only 
one factor may be varied at a time. All other factors 
must be held constant throughout the tests. Usually 
more than one characteristic is to be measured, so 
that more than one test must be made. The above 
rule, that all factors except the one under test must be 
kept constant, applies to the entire testing program. 
A few illustrations which apply to underwater sound 
testing follow. 

It is essential that all characteristics of the medium 
remain constant throughout the tests. For example, 
temperature: unless it is a variable of the test, the 
temperature must be uniform throughout the pro- 
gram. In addition, the device itself must be in tem- 
perature equilibrium with the medium. Depending 
on its size and type of construction, this may require 
waiting several hours or even a full day, while the 
unit is immersed, before tests can be started. It has 
also been found that results obtained at one tempera- 
ture do not necessarily apply at other temperatures. 
Thus the temperature of the water during the tests, 
the temperature dependence of the device, and the 
speed with which it reaches temperature equilibrium 
are important factors. 

In any extended testing program, special care must 
be taken that the signal levels, the transmission prop- 
erties of the medium, and the noise background do 
not change. Drifts in the amplifier or in the oscillator 
characteristics affect the acoustic conditions by chang- 
ing the level or the frequency of the sound in the 
water. To avoid such drifts, it is necessary to check the 
electric system frequently during the tests. Similar 
precautions must be observed in using acoustic de- 
vices. For instance, x-cut Rochelle salt crystals are 
variable under some conditions and for this reason 
are undesirable as standards. In many devices one side 
is grounded, increasing the chance for noise pickup 
and necessitating proper shielding of the leads. 

Another factor to be considered is that all measure- 
ments must be made at the same point in the circuit, 
that is, the leads should be the same for all tests. The 


18 


TRANSMITTING 


19 


same auxiliary equipment, such as tuning coil or con- 
denser and polarizer, should also be used throughout 
the testing program. For example, in the measure- 
ment of electric power, the current and impedance, or 
current, voltage, and phase angle must be measured 
from the same set of terminals. In more complicated 
tests the matter of measuring from a single pair of 
terminals, however, is sometimes overlooked. For in- 
stance, in order to determine the efficiency of a pro- 
jector, the transmitting response must be obtained, 
the directivity measured, and the impedance of the 
projector and of the source determined. To obtain 
the correct answer, all electric measurements that 
enter into these tests must be made from the same ter- 
minals. Frequently this is inconvenient. The driving 
amplifier, for instance, is at one place in the labora- 
tory, while the impedance bridge is at another. If an 
extra lead is added in either connection, an error 
results. 

It may be noted in passing that the efficiency thus 
determined includes any losses in the system beyond 
the point where the measurements are made. If the 
efficiency of the projector exclusive of these losses is 
desired, the measurements must be made directly at 
its terminals or the losses must be eliminated from the 
data by computation. The latter method is usually 
more time-consuming and less accurate. 

A general principle of testing is that it is easier to 
make relative measurements than absolute measure- 
ments, and that the precision of relative tests, for the 
same amount of effort, is much greater. The easiest 
tests to make are the so-called /I -5 comparisons which 
involve immediate switching between two conditions. 
Usually one of the conditions, say A, which is well- 
known, serves as the reference condition, and the 
other, B, which includes the unknown, is the test con- 
dition. Many a testing difficulty can be avoided by re- 
ducing the program to a number of such comparison 
tests, and, if at all possible, tests should always be set 
up on that basis. 

With these principles in mind, many specific prac- 
tices have been established by USRL. Some of these 
are described in Section 6.1.4. The rest of this section 
is concerned with the physical characteristics to be 
measured in calibration tests on underwater sound 
equipment. 

Most acoustic devices are reversible, that is, they 
can do two things depending on how they are used: 
(1) When an electric voltage is applied at their ter- 
minals, they generate acoustic power, and (2) when 


acoustic power is supplied to them, they generate an 
electric voltage. A device which has these properties 
in the underwater sound field is called a transducer. 
The first action is called transmitting and the second 
receiving. Certain conventions**' have been set up by 
agreement among the different groups interested in 
the underwater sound field for the measurement of 
transmitting and receiving performance. These con- 
ventions are of value in making the meaning of meas- 
urement results precise to all people in the group. 
Also, by expressing all results on the same basis, dif- 
ferent measurements can be more easily compared. 
The most important use of test data is usually to 
determine which instrument is best from among a 
number that are available for a particular application. 

41 TRANSMITTING 

Transmitting measurements usually involve three 
factors: (1) the acoustic pressure delivered by the de- 
vice in the desired direction for a known electric in- 
put, (2) the distribution of the acoustic pressure in 
other directions, and (3) the variation of the pressure 
with frequency. 

It is noted that these items correspond respectively 
to (3), (1), and (2) at the beginning of this chapter. 

These quantities must be expressed in such a way 
that their meaning is unambiguous and that they af- 
ford a ready means of comparing different designs. 
For example, a statement of the pressure delivered 
does not of itself tell much about the performance of 
a device, since it is possible to change the pressure by 
increasing or decreasing the electric input. Conse- 
quently, the electric supply conditions must be speci- 
fied as well. 

In most practical cases the projector is fed from an 
amplifier. The most definite way to tie down the elec- 
trical system in a practical way, therefore, is to specify 
the amplifier. For calibration purposes it is desirable 
that a class A amplifier be used, because the perform- 
ance of such an amplifier can be accurately specified 
and controlled. From the standpoint of circuit analy- 
sis, a class A amplifier can be replaced by a generated 
voltage Cg and an internal resistance Vg. The same 
analysis also applies to class B and C amplifiers, but 
the values then are a function of the power delivered, 
whereas in the case of the class A amplifier these 

a Conference on underwater sound projectors in the Office 
of the Coordinator of Research and Development of the Navy, 
July 19, 1944. 


20 


TYPES OF ACOUSTIC MEASUREMENTS 



Figure 1. Circuit referred to by equation (1). 


values are independent of this factor up to the point 
where overloading sets in. 

It is seen from the above that the power delivered 
by an amplifier is a function of the load impedance. 
For this reason, the use of a fixed input power, a hxed 
applied voltage, or a fixed applied current in deter- 
mining the variation with frequency of the pressure 
delivered by a projector (the impedance of which 
changes with frequency) in general does not provide a 
response characteristic that is representative of actual 
service conditions. This consideration has led to the 
use of available power as a basis. 

* Available Power 

Available power^" is defined as the power which a 
driver having a hxed generated voltage eg and a hxed 
internal resistance Vg delivers into a matched load re- 
sistance Figure 1 illustrates the circuit. 

From this circuit it can be seen that the power de- 
livered into the load resistance = Vg is 



curves of this transition loss are plotted against the 
ratio of the impedance magnitudes Vgjz for different 
phase angles 0. 

If a projector is tuned, so that at resonance its im- 
pedance is a pure resistance, the maximum power 
will then be delivered by the driver to the projector 
if this resistance matches the internal resistance of 
the driver, that is, when = Vg. It is seen that under 
these conditions, the actual power Pj equals the avail- 
able power T 4 . In all other cases the actual power is 
less than the available power. The efficiency of the 
electric system under these conditions, however, is 
only 50 per cent, since the amount of power dissipated 
in the output tubes equals that supplied to the load. 
In practical designs using class B or C amplifiers, it 
is an ad\’antage to use a lower source impedance, 
about 1/4 i'l- This improves the electric circuit effi- 
ciency and reduces the power dissipation in the out- 
put tubes, thus permitting smaller tubes to be 
employed. In testing, the source impedance of the 
actual system should be simulated. 

2 Transmitting Response 

The transmitting response of a projector of given 
impedance is expressed in terms of the pressure at 1 
meter distance on the acoustic axis in decibels versus 
reference pressure (1 dyne per sq C7n ) per watt avail- 
able poxoer frotn a given generator impedance (as- 
sumed to be purely resistive). 

In connection with this definition of transmitting 
response, it should be noted that the pressure de- 
livered increases as the square root of the available 



The term 10 logP^/P^ is well-known in transmis- 
sion circuit theory^^ and is the transition loss between 
the resistance and the impedance z. On Figure 2 


Figure 2. Fransition loss between a generator having an 
internal resistance of ohms and a load impetlance of 
magnitude z and phase angle d. 



TRANSMITTING 


21 


power. Thus, formulating the mathematical expres- 
sion for the response, 

Rrp = 20 log - = 20 log p — 10 log P4 . (3) 

\/Pa 

In equation (3), 20 log p gives the pressure in deci- 
bels versus 1 dyne per sq cm (1 dyne per sq cm is called 
reference pressure) and 10 log is the power level 
referred to 1 watt. For actual testing, the practice of 
expressing power levels in decibels versus 10~^^ watt 
has developed. 

It will be noted that 1 meter is chosen for the refer- 
ence distance. This, of course, does not mean that all 
calibrations are to be made at that distance. The ac- 
tual testing distance will depend on considerations of 
obtaining waves which are sufficiently plane so that 
spherical wave corrections will not be required either 
for the projector under test or for the receiving hydro- 
phone, that is, the testing distance will depend on 
the size of the instruments and the frequency. Other- 
wise, the testing distance will be made as short as pos- 
sible to minimize interference from reflections, etc. 
The testing distance d will be stated in connection 
with all tests and the correction C in decibels to d^) = 

1 meter will be made on the basis of spherical waves; 
thus 

C = 201og^- (4) 

do 

A similar consideration applies with respect to the 
power to be used in the tests. While the response is 
referred to 1 watt, it would obviously be incorrect to 
make all tests at that power level. If the device is 
linear, the testing power used is of no consequence, 
but if the response varies with power level, then the 
tests should be made at the actual working levels used 
in service and the testing power should be stated. A 
load characteristic should be furnished showing the 
relation between acoustic power output (or pressure 
on the axis) and available power. 

Directivity 

The next item to be measured concerns the distri- 
bution of the acoustic pressure with direction. This 
is measured by determining the pressure over a spher- 
ical surface having the projector as the center, the 
pressure p in any one direction being expressed in 



Figure 3. Three-dimensional directivity pattern for a 
circidar plate. Frecjuency = 25 kc, diameter of plate = 15 
in. Decibel values shown give response relative to that on 
normal axis. 

decibels versus the pressure pQ 011 the acoustic axis of 
the device. For the acoustic axis an axis of symmetry 
of the device is usually chosen, which frequently is 
also the direction of maximum response. A plot of 
these values is called a directivity pattern. A view of 
a three-dimensional directivity pattern for a circular 
plate is shown in Figure 3. For devices which are sym- 
metrical, such as a circular plate, the directivity is the 
same in all planes containing the major axis normal 
to the surface. Thus the pressure distribution need be 
measured only in one plane, resulting in a great re- 
duction of work. A planar directivity pattern is 
shown in Figure 4. This is the usual way of plotting 
these patterns. Devices which are not circular usually 
have several major axes. Patterns should then be 
taken in all planes containing one of these axes. 

The directivity index is defined as the ratio in deci- 

bels of the intensity I (I = — far away from the source) 

averaged over all directions to the intensity /q on the 
acoustic axis of the device: 

A = 1 0 log — = 20 log ^ . (5) 

h po 


22 


TYPES OF ACOUSTIC MEASUREMENTS 


0 * 



Figure 4. Planar directivity pattern for a circular plate. 
Frequency = 25 kc, diameter of plate = 15 in. 


There are several cases for which a directivity in- 
dex can be obtained in a relatively simple manner.^^ 
If the directivity pattern of the transducer has rota- 
tional symmetry about the acoustic axis, one may 
make use of the following formula 

A = 10 logio ^ sin a rfaj. (6) 


is 24 'db below the peak. Therefore 10 log /q/^o = 
— 24 or /a//o = 0.004. The sine of 25 degrees is 0.26. 
Consequently la/^o sin a = 0.00104. The values of 
la/^o sin a are computed for all angles and plotted on 
rectilinear graph paper against the angle a expressed 
in radians (1 radian = 57.3 degrees). This is illus- 
trated in Figure 5. The area under the curve is meas- 
ured with a planimeter and is found to be 5 square 
inches. In this particular case the scales were chosen 
so that 1 inch on the abscissa represents 0.1 radians 
and 1 inch on the ordinate represents an intensity 
ratio la/^o = 0.01. Thus, 1 square inch represents a 
contribution to the integral of 0.001. Hence, the total 
area gives 

I ^ sin ada = 0.001 X 5.00 = 0.005. 

Jo ‘0 

Equation (6) includes a factor of I /2 in front of the 
integral. Thus the directivity factor is 0.0025 and the 
corresponding directivity index A is 

A = 10 log 0.0025 = -26 db. 


In this formula, a represents the angle from the 
acoustic axis, 1^ the intensity at this angle, and /q the 
intensity on the axis. The above formula is valid, for 
example, in the case of a circular diaphragm vibrat- 
ing symmetrically about this normal axis. In the case 
of a line source, the acoustic axis is usually taken 
perpendicular to the line. If the directional pattern 
of the line is symmetrical about the line itself, the 
directivity index is given by the formula 


A = 



( 7 ) 


where a is now the angle measured from the acoustic 
axis (normal to the line) in a plane including the 
acoustic axis and the line itself. 

To indicate the use of these formulas, consider the 
pattern shown in Figure 4, representing a measured 
pattern for a circular piston in a plane including the 
acoustic axis. To find the directivity index for the 
pattern, equation (6) above is applied. The integral 
is evaluated graphically by means of a planimeter. 
This requires obtaining for different angles the 
ratio /a//o- For instance, at 25 degrees the response 


The procedure in the case of a line is analogous to 
the one described above, except that cos a is used in 
all cases in place of sin a, as indicated by comparison 
of equations (7) and (6). 

Sometimes the pattern as obtained experimentally 
is not exactly symmetrical. In that case, it is usually 
sufficiently accurate to use the average values of the 
two halves of the pattern obtained experimentally. 

This computation is quite straightforward but 
somewhat tedious. Figure 6 shows a chart which has 
been prepared to reduce the amount of algebraic 
computation involved. This chart shows a family 
of curves, each curve corresponding to a particular 
value of /a/7o sin a. The chart is used in conjunction 
with the directivity pattern, plotted on polar coordi- 
nate paper, of the instrument whose directivity index 
is to be obtained. The use of the chart is as follows: 
The transparent chart is laid over the directivity pat- 
tern of the instrument so that the coordinate systems 
on the two charts coincide. Then, to find the value 
of lajh sin a for any angle a, one proceeds along the 
radial line corresponding to the angle a until one 
reaches the intersection of that line with the direc- 
tivity pattern of the instrument. The point of inter- 
section of the pattern and the line will then fall on 



TRANSMITTING 


23 



Figure 5. Computation of directivity index for a circular plate vibrating at uniform amplitude and in phase. 
Frequency = 25 kc, diameter of plate = 15 in. 



24 


TYPES OF ACOUSTIC MEASUREMENTS 



Fk;ukf, 6. Directivity index calculator. 










TRANSMITTING 


25 


one, or between two, of the curves of the family of 
curves drawn on the chart. Suppose, for example, 
that this point falls about 7/10 of the way between 
the curves corresponding to 0.003 and 0.004. Then 
for the angle a 

^ sin a = 0.0037. 

The values of la/Io sin a may be quickly obtained for 
each desired value of the angle a. These are then 
plotted as described above. The procedure beyond 
this point is identical with that described earlier. 
The use of this chart at USRL has indicated that it 
is as accurate in general as a direct computation and 
the work proceeds much more quickly, particularly 
since most field data for instruments give directly 
10 log la/Io rather than la/h- 

In the above, it is assumed that the pattern has 
been drawn with the maximum on the outer circle on 
the coordinate paper. If the maximum is drawn on 
the circle which is 10 db down, the chart may still 
be used in the same way, but the values obtained for 
la/ J q sin a from the chart should be multiplied by 10 
to obtain the correct values; if the maximum is on 
the circle 20 db down, the chart values must be mul- 
tiplied by 100, etc. 

The chart can also be applied to a line. In this case 
it is turned so that 90 degrees on the chart coincides 
with 0 degrees on the directivity pattern. Since the 
chart in that direction is narrower, it will be neces- 
sary to plot the pattern on a smaller scale in order 
that the chart may accommodate it. 

If the beam width is not too broad (total beam 
width 10 db below peak does not exceed 120 degrees), 
and the side lobes and rear response are at least 15 db 
below the peak, the directivity index is practically 
determined by the beam width alone. Thus, in Figure 
7 the directivity index is plotted for a circular plate 
as a function of the beam width. By referring to this 
chart, one may read directly the directivity index for 
the measured beam width. 

Many devices do not have directivity patterns sym- 
metrical about a single axis. In general, then, direc- 
tivity patterns would have to be measured in a great 
many planes passing through the acoustic axis, and 
a laborious double numerical integration performed 
to obtain the directivity index. In some cases where 
the pattern is symmetrical with respect to two per- 
pendicular planes passing through the acoustic axis. 



Figure 7. Directivity index as a function of the beam 
width for a circular plate. 


an approximate value for the directivity index may 
be obtained by a simple procedure which requires 
taking directivity patterns only in these two planes. 
It is further required that the beam widths in these 
two planes be less than 120 degrees. To illustrate this 
method, let us consider a rectangular piston which 
is the most commonly occurring nonsymmetrical type 
in practice. For such a piston, the directivity index 
given is directly determined only by the beam width 
in the two planes and can be represented as a function 
of this beam width. For this case, a chart is given in 
Figure 8, from which the directivity index can be 
found from the measured beam width, 10 db down, 
in the two planes. The two planes in this case are the 
the planes passing through the acoustic axis and pa- 
rallel respectively to the two pairs of sides of the rec- 
tangle. A similar calculation can be made for an 
elliptical piston when the two planes are taken 
through the acoustic axis and parallel respectively 
to the major and minor axes of the ellipse. A chart 
for this case also is given in Figure 8. These charts 
may be used in conjunction with measured patterns 
for rectangular or elliptical transducers. 

Consideration has been given to reduction of side 
lobes by means of tapering. (See reference 14.) By 
tapering is meant the variation of the velocity distri- 
bution over the diaphragm of the transducer so that 
the velocity decreases from the center to the peri- 
phery. This method is quite effective for reducing 
side lobes, but it has the undesirable effect of increas- 
ing the beam width. The methods described above 
for calculating the directivity index can in general be 
applied directly to tapered transducers. The effect of 


26 


TYPES OF ACOUSTIC MEASUREMENTS 


MINIMUM BEAM WIDTH (10 OB DOWN) 



Figure 8. Directivity index as function of beam widths for rectangular and elliptical pistons. Maximum beam widths 
are measured in the plane through the acoustic axis parallel to the short side of the rectangle or including the minor 
axis of the ellipse. Minimum beam widths are measured in the plane through the acoustic axis parallel to the long 
side of the rectangle or including the major axis of the ellipse. 


tapering on the directivity index, however, except in 
extreme cases, is relatively small.*^ 

The following additional information can be ob- 
tained from the directivity pattern: 

1. The angle of maximum response is the angle be- 
tween the direction of maximum response and the 
acoustic axis. 

2. Beam width may be defined as the angular sepa- 
ration between the two points on either side of the 
main beam which are 10 db below maximum. 

3. Height of side lobes may be expressed in terms 
of the maximum pressure in any direction within the 
side lobe in decibels versus the pressure on the axis. 

4. Rear response is defined as the maximum pres- 
sure within ±60 degrees from the rear in decibels 
versus the pressure on the axis. 

It should be noted in this connection that, pro- 
vided the device is linear, the directivity patterns 
and the directivity indices for transmitting and for 

b It has been shown by E. Gerjuoy that the directivity index 
of a circular plate has its optimum value when no tapering is 
used. 


receiving are identical at each frequency. This fol- 
lows from the reciprocity principle. 

Projector Efficiency 

Another criterion which can be derived from the 
response and directivity measurements is of special 
interest to the designer because in the most funda- 
mental way it rates his design as an electric motor de- 
livering acoustic power for the electric power sup- 
plied. This criterion is the projector efficiency, de- 
fined as follows: 

The projector efficiency is the ratio in decibels of 
the total acoustic power delivered by the projector to 
the electric power input into the projector. 

To compute the efficiency Ep of the projector it is 
necessary to know the transmitting response i?r, the 
directivity index A, and the projector impedance z. 
I'he projector efficiency^® then is given by 

£, = At + A - 10 log|i. - 70.9. 


( 8 ) 


RECEIVING 


27 


In connection with these measurements, reference 
should be made to the discussion at the beginning 
of this chapter where are stated certain precautions 
that must be observed in taking the data in order to 
obtain a precise determination of the efficiency. 

It is shown later in this chapter that, when the 
device is linear, the projector efficiency is the same 
on transmitting and on receiving. This further illus- 
trates the fundamental nature of this quantity. 

Selectivity 

The above quantities relate to the transmitting 
performance of the device at any one frequency. In 
general, however, the performance over a range of 
frequencies is of interest. In that case, the response 
and efficiency are determined over the frequency 
range and plotted versus frequency to provide re- 
sponse or efficiency characteristics. Directivity pat- 
terns may also have to be taken at several frequencies. 

The response characteristic in particular is used to 
study the selectivity of the device. For this purpose 
use is frequently made of the equivalent series reso- 
nant circuit. This circuit is one in which the current 
varies with frequency in the same way that the re- 
sponse does. Assume this circuit to have resistance r 

and reactances Lw and -i- (where w = 27r/). Its impe- 
Co) 

dance then is 



Since at resonance = 1 jLC, 


For constant applied voltage e, the currents then are 

and /q = -• 


— 




Hence 


20 log U = 20 log ^2 = 20 log 3 db. 

^0 ^0 \/2 


The symbol Q has been used for the ratio Lwo/r. 
Usually in a resonant circuit the resistance is asso- 
ciated with the coil. The Q of the coil then is its qual- 
ity or figure of merit. To obtain the Q of a response 
curve, first the frequencies h and fo, at which the re- 
sponse is 3 db below the peak, and the resonant 
frequency /q are found; then Q is found from these 
three frequencies by means of the above relation: 


0 .= 


fo 

fl — /2 


( 9 ) 


42 RECEIVING 

An underwater acoustic device which is used only 
for receiving is called a hydrophone. The measure- 
ments on receiving usually involve determination of 
the following factors: 

1. The voltage delivered by the device. This is re- 
ferred to the condition in which the unit is in a 
uniform, plane sound field of reference pressure 
(1 dyne per sq cm). 

2. The variation of this voltage with direction of 
sound incidence. 

3. The variation of the voltage with frequency. 


z = r -f ; L(o 

w 

The ratio of the resistance r to the reactance x in 
this circuit is 



There are two values, wi and W 2 , one on each side of 
resonance wo, where x = r. Then |zi| = jzsl = r\/2. 
At resonance, that is, at wq, we have x = 0, so that 
Zo = r. In addition because of symmetry coq = V o>ia>2) 
SO that 

^<^0 _ top _ fo 

r 0)1 — 0)2 fl — f 2 


These factors are analogous to those tested on trans- 
mitting. There is another quantity of interest on re- 
ceiving, the threshold pressure. This denotes the pres- 
sure on the face of the hydrophone that generates a 
voltage equal to its inherent noise voltage. This is re- 
lated to the minimum signal that can be measured 
with the particular instrument. Of these quantities 
only the receiving response and the threshold are con- 
sidered in detail, since the other items have been 
covered on transmitting. 

Receiving Response 

The receiving response of a hydrophone or a pro- 
jector is expressed in terms of the open-circuit voltage 


28 


TYPES OF ACOUSTIC MEASUREMENTS 


in decibels versus 1 volt, generated by the unit in a 
unijorni plane-ivave, free sound field of reference 
pressure (1 dyne per sq cni) propagated parallel to the 
acoustic axis of the hydrophone. 

The selection of the open-circuit voltage has the ad- 
vantage that, with a few exceptions noted below, it is 
possible to compute the signal voltage across any load 
impedance when the open-circuit voltage Cg and the 
impedance z of a projector or a hydrophone are 
known. For instance, if the load impedance is z^, the 
voltage across it is 


For certain types of hydrophones, designed for spe- 
cial purposes or including a preamplifier of the cath- 
ode-follower type, it is desirable to state the closed- 
circuit voltage instead of the open-circuit voltage. 
The load impedance across which the voltage is meas- 
ured must be stated in all such cases. 

In order to measure the open-circuit voltage, a very 
high impedance circuit is required, especially when 
dealing with crystal hydrophones, which have high 
impedances themselves. Frequently the measure- 
ments are made in a closed circuit and then the cir- 
cuit loss is allowed for. In hydrophones which have a 
preamplifier associated with them, a small resistance 
is frequently included in the so-called calibration cir- 
cuit to permit computing the open-circuit voltage 
generated by the crystal. Care must be taken in con- 
nection with the measurement of the response of such 
hydrophones that the output of the preamplifier is 
properly terminated. This applies especially to the so- 
called cathode-follower circuit which is commonly 
used. 

Formulating a mathematical expression for the re- 
ceiving response, we have 

7J„ = 201og^ , (10) 

where Cg — the generated voltage of the hydrophone 
in volts, and p = the pressure in the free field sound 
field in dynes j)er S(j cm. 

Instead of obtaining the open-circuit voltage, the 
group at the Woods Hole Oceanographic Institution 
j)refer to calibrate their tourmaline gauges in terms of 
the electric charge on the crystal. This calibration is 
made by comparing the voltage Vp across the ampli- 
fier input (or output) due to a known pressure on the 


crystal, with the voltage Vq across the amplifier input 
(or output) due to a calibrating voltage Vg applied 
across the amplifier input in series with a known cali- 
brating condenser Cg. The charge on the crystal due 
to the applied pressure can then be computed from 
the above mentioned voltages and the calibrating ca- 
pacity. If the calibrating condenser Cg is in parallel 
with the crystal when the voltage Vp is measured, 

Q _ f p Vs 
f c 

From this charge Q and the capacity of the crystal Cq, 
the open-circuit voltage e can then be found. 

Q = 

The determination of the charge instead of the gen- 
erated voltage is convenient at times because it re- 
quires only measurements at the input or output of 
the amplifier, and the results are independent of the 
length of the intervening cable. 

^ 2 ^ Threshold Pressure 

The threshold of a hydrophone or a projector zuill 
be expressed in terms of the pressure in a uniform, 
plane-iuave, free sound field propagated parallel to 
the acoustic axis of the device, in decibels versus refer- 
ence pressure (1 dyne per sq cm), which produces a 
signal voltage equal to the inherent noise voltage. 
This noise voltage is taken in a band luidth of 1 cycle 
and the device is supposed to be in a matched, tuned 
circuit. 

In the following, the significance of the term thresh- 
old is discussed and the measurements and computa- 
tions necessary to obtain the threshold pressure are 
outlined. 

The signal pressure which can be measured with 
any given device is limited in two directions: over- 
loading limits it on the upper side, and the noise level 
limits it on the lower side. The noise may be due to a 
number of factors, such as the associated preamplifier, 
pickup in the leads, improper grounding, and con- 
tacts. When these sources are eliminated there re- 
mains thermal noise,"^^* which is fundamental and 
depends only upon the temperature and the fre- 
quency range covered. The mean s(|uare value of this 
random noise voltage has been determined experi- 
mentally and theoretically. AVhen reduced to a fre- 


RECEIVING 


29 



Figure 9. Circuit equivalent to a long line. 


The rms noise voltage applied to the grid due to 
both and 6^2 is obtained by adding the two fluc- 
tuating noises at random phase, thus 



+ ^n2 


/_j_Y 

+ rj 


( 12 ) 


When the line impedance is matched at the two ter- 
minals, 7 'rp = r. Then by equation (11) above, e^i = 
^n 2 = so that 




quency band of 1 cycle and a temperature of 20 C it 
has the value 


= 1.61 X 10-2Or„ (11) 


where is the equivalent series resistance of the 
device. 

In the practical case there are two distinct condi- 
tions to be considered: calculated threshold and meas- 
ured threshold. 

Calculated Threshold 

In the case of low-impedance hydrophones, usually 
of the electrodynamic or magnetostrictive type, the 
active unit usually is directly connected to a line. 

If this line has appreciable length, its impedance 
must be matched at both terminals, since otherwise 
irregularities in response are introduced due to reflec- 
tions in the line itself. Neglecting attenuation, the 
circuit may then be represented as in Figure 9. 

Here eg is the generated open-circuit signal voltage, 
Cni is the generated noise voltage in the resistance of 
the hydrophone r, and 6^2 is the generated noise volt- 
age in the terminating resistance ?>. Reactances in the 
circuit are omitted for the sake of simplicity. is the 
noise voltage and Vg is the signal voltage applied to 
the input of the measuring circuit, assumed to be the 
grid of the amplifier. 

The noise voltage applied to the grid due to is 



(13) 


The signal voltage Vg applied at the grid is then one- 
half the voltage generated by the hydrophone. 



From the above relation Vg, the signal voltage gen- 
erated by the hydrophone, equals F„, the noise volt- 
age in the matched circuit, when 

1.41c,. (14) 


The other possibility, instead of matching the circuit, 
is to connect the hydrophone to a very high impe- 
dance. In that case is very large relative to r(rj,-^oo). 
Hence from equation (12) 

(15) 

and the signal voltage Vg applied to the grid becomes 

SO that 

= c,i. (16) 



The noise voltage applied to the grid due to e „2 is 



It is seen by comparing equations (14) and (16) that 
there is a theoretical gain in the signal-to-noise ratio 
of 3 db in terminating the hydrophone in a high im- 
pedance as compared to matching it. Where the leads 
are sufficiently short it is therefore advantageous to 
use a high-impedance termination. This applies espe- 
cially to the internal connection between the active 


30 


TYPES OF ACOUSTIC MEASUREMENTS 


unit and its preamplifier, a case which is discussed 
below. 

The threshold pressure for low-impedance hydro- 
phones may be computed from the receiving response 
Rji and the resistance of the hydrophone r. This rela- 
tion is as follows: 

Substituting in the above equation (14) for the 
value given by equation (11) leads to the results 

e, = 1.79 X 10-1%/^. (17) 

The signal voltage is related to the signal pressure p 
(in dynes per sq cm) by means of the receiving re- 
sponse Rr, in accordance with equation (10), 

«« = 201og(^). 

which may be written in the form 

20 log p = 20 log - Rr. 

Introducing in this equation the signal voltage de- 
fined by equation (17), we obtain the threshold pres- 
sure in decibels 

r = 20 log /> = 20 log (1.79 X 10-i»v^) - Rj^ 

= 10 log r- 194.9 - (18) 

The test procedure in accordance with the above is 
to measure the resistance and response of the hydro- 
phone. From these values the threshold pressure then 
is computed by means of equation (18). 

Measured Threshold 

For a high-impedance hydrophone of the crystal 
type, the active element is usually directly associated 
with a preamplifier. This is necessary in order to 
avoid excessive losses in the leads and also to pre- 
vent noise pickup, to which a high-impedance circuit 
is apt to be subject. Frequently the preamplifier is 
given an extremely high input impedance. This is 
done in order to obtain the maximum signal-to-noise 
ratio at the first grid (as discussed) and also in order 
to stabilize the hydrophone. For instance, in the case 
of x-cut Rochelle salt crystals, which are inherently 
variable with temperature, the variability is reduced 
when no current is drawn from them. 


Since the preamplifier is so intimately associated 
with the active element, it is best to treat the two as 
one unit and to determine the threshold for the com- 
bination. As a rule, the noise of the preamplifier ex- 
ceeds the thermal noise to such an extent that the 
latter has little practical importance. The computa- 
tion outlined above then becomes useless and the 
only practical way to proceed is actually to measure 
the inherent noise level of the instrument. This re- 
quires an extremely quiet body of water and a very 
quiet measuring system in order that extraneous 
noise does not enter into the tests. When quiet water 
is not available, a possible alternative is to substitute 
a network for the crystal. The latter must be the elec- 
trical equivalent of the crystal in water over the entire 
frequency range included in the measurement. The 
measuring system, in addition, should have uniform 
response and a definitely defined band width, narrow 
enough so that variations in the hydrophone response 
within the band may be neglected. Usually the pre- 
amplifier output is matched. If the measuring band 
includes the frequencies from fi to / 2 , then the value 
10 log (/2 — /i) must be subtracted from the measured 
noise voltage (assuming it to be in decibels), in order 
to obtain the threshold. 

4 3 RELATIONS BETWEEN 

MEASUREMENTS 

In the following, certain relations that exist be- 
tween the measured quantities are pointed out.^^ 
These relations frequently are useful in cross-check- 
ing measurements. They also reveal additional in- 
formation as to the nature of the definitions. 

1. The relation between the projector efficiency 
Ep and the transmitting response R^ was given 
in equation (8): 

£, = + A - 10 logg - 10 log 

= fir + A - 10 log ^ - 70.9. 

2. There was also given the relation between the 
calculated threshold and the receiving response 
Rr in equation (18) 


T = 10 log r- 194.9-/?^. 


RELATIONS BETWEEN MEASUREMENTS 


31 


3. There exists, in addition, a reciprocal relation‘s 
between the transmitting response and the re- 
ceiving response of a projector. This relation 
has the following form: 

iJr = fl« + 201og/+ lOlog^ 

- lOlogr + 20 log 


the study of measured data. For the sake of simplicity 
the discussion is confined to circular pistons. 

In the case of a circular piston moving rigidly in an 
infinite baffle, there exists a simple relation between 
its radius a and the directivity index 


A = 


— 10 log 


, _ 2/i {2ka)-X 

2ka J 


(23) 


= + 20 log / + 10 log ^ — 10 log r + 94.2. 

“a 

(19) 

4. From these three equations, it is possible to de- 
rive a relation between the projector efficiency 

and the threshold T : 

£p + r = A + 20 log/ + 10 log^:^ - 194.9 

= A + 20 log/ + 171.6. (20) 

5. Furthermore, by combining equations (18) and 
(19) or (8) and (20), a relation can be obtained 
between the threshold T and the transmitting 
response of the unit Rrp. This relation is as fol- 
lows: 

Rr+T= 10 log + 20 log/ 

' A 

+ 20 log - 194.9 
= 10 log ^4 + 20 log/- 100.7. (21) 

T A 


where k = 27r/A, A being the wave length = c//, and 
Ji = first order Bessel function. 

The directivity of a physical projector usually is 
less than that of a theoretical circular piston of the 
same geometrical size. It is, however, possible to de- 
fine the effective or acoustic radius of the physical 
projector to be the radius of the theoretical piston 
having the same directivity index as the projector. On 
Figure 10 the directivity index and beam width are 
plotted against effective radius in wave lengths (a/\) 
for a theoretical piston. 

A number of interesting relations are obtained by 
introducing this expression in the above equations in 
which the directivity index occurs. These are the 
equations which include the projector efficiency. For 
instance, substituting the above expression for the 
directivity index in the relation between projector 
efficiency and transmitting response, equation (8) 
gives: 

2/i {tka)l 

2ka J 

+ 20 log A -10 log 



6. Finally, by combining equations (8) and (19) a 
relation can be found between the projector 
efficiency and the receiving response of the de- 
vice: 


= Rj< — 10 log 


2/i (ika)-\ 

2ka J 



= £« + A + 20 log / — 10 log r + 10 log 

= fiB+ A + 201og/ - lOlogr + 23.3. (22) 

The following discussion has for its purpose the ex- 
ploration of the meaning of the above relations and 
the indication of their usefulness in connection with 


c The reciprocity theorem and the conditions under which it 
applies are stated in Chapter 3. 


+ 20 log A - 81.9. (24) 

TTOr’ 

In the term r 2/i i2ka)~\ this equation, ira^ is 

the area of the theoretical piston of radius a. In the 
case of an actual projector, in accordance with the 
above discussion, a is the effective radius and can be 
found by means of the chart on Figure 10. The ex- 
pression in the bracket is a pure numeric, so that the 
whole term has the dimensions of an area. Let it be 


32 


TYPES OF ACOUSTIC MEASUREMENTS 



Figure 10. Relation between effective radius and direc- 
tivity index (or beam width) for sonar projector with 
circular diaphragm. 


and write 

_ 10 log^ + 20 log A + K^. (27) 

This expression shows that for constant projector 
efficiency, the transmitting response varies directly 
as the effective area, and for any given device {A = 
constant) increases 6 db per octave. The latter rela- 
tion is shown by equation (21) to exist also for fixed 
threshold and transition loss. 

Introducing the expression for the directivity in- 
dex in equation (20) gives 

E^+T = -lOlog^ + 10 log - 194.9 

= -lOlog/4 - 79. (28) 


called the effective area of the projector^ A, which is 
thus defined 

L 2ka J 

Defining the directivity factor 8 by 
A = 10 log 8, 

it is evident from equation (23) that 


This equation shows that for any given device 
(A = constant) the threshold pressure is independent 
of frequency and also indicates that the higher the 
efficiency of the projector the lower the threshold 
pressure. For devices of the same type, that is, having 
the same efficiency, the threshold varies inversely as 
the size of the unit. 

Substituting the above expression for the directiv- 
ity index in equation (22) we obtain 

Ep = Rji — \0 log .4 — 10 log r + 1 0 log 


8 = 


2/i i2ka) l 

2ka J 




and from equation (25) that 



(26) 


In case a exceeds one-half wave length, the term 
1 — [2/i {2ka)/2ka] is nearly unity, so that A then 
equals na^. 

Assuming, furthermore, the transition loss to re- 
main unchanged, we can simplify equation (24) by 
introducing a constant K^, 

K, = -(lOlog^ + 81.9), 

<1 The effective area was first defined in these terms by E. E. 
Teal of the Columbia University Underwater Sound Laboratory 
at New London in a letter dated February 23, 1943. 


= Rr- 10 log .4 - lOlogr-f 115.9. (29) 

From this equation it may be seen that for a given 
device having fixed size and efficiency, the receiving 
response is independent of frequency but increases 
directly with the resistance. Advantage is often taken 
of this latter fact by using a step-up transformer to 
increase the receiving response of a low-impedance 
hydrophone. These relations between receiving re- 
sponse and impedance are shown by equation (18) to 
apply also for a fixed threshold. It is also interesting 
to note that for a given receiving response and effi- 
ciency the resistance among different projectors varies 
inversely as their area. 

It is, of course, possible to set up a definition for 
the receiving efficiency of a hydrophone or a projec- 
tor. In terms analogous to those used for the projector 
efficiency given above, this efficiency could be stated 

Ek = 10 log^- 


RELATIONS BETWEEN MEASUREMENTS 


33 


In this equation, the electric signal power delivered 
by the hydrophone in a matched circuit is 



For the acoustic input power it is possible to take the 
product of the free field intensity, p^j pc, and A, the 
effective diaphragm area, and obtain, 

h- 

Pj = ^ ^ X 10^ watts, 
pc 


the medium and is sufficiently large so that diffrac- 
tion can be neglected. 

Introducing these values in the above equation for 
Ejf, we obtain 

E„ = R,f - lOlog/1 - 101og)-+ lOIogC!^. 

(30) 

Comparing this expression with equation (29), 

Ep = Rji — \0 log A — 10 log r + 10 log , 


where p is the free field pressure. Pj is actually the 
available acoustic power in the water, which equals 
the actual acoustic input power if the device matches 


the important result is obtained, that Ej^ = E^. Thus, 
a projector has the same efficiency on transmitting 
and on receiving. 


Chapter 5 ^ 

TESTING TECHNIQUE 

By Leslie L.Foldy 


5 1 THE TESTING PROBLEM IN 
GENERAL 

Calibration and Operational Testing 

T he testing of underwater sound devices assumes 
two forms, depending upon the type of informa- 
tion desired. In one type of test, it is desired to obtain 
information which characterizes the device independ- 
ent of its environment to such an extent that its be- 
havior in any particular environment can be pre- 
dicted. Such a test is known as a calibration test. On 
the other hand, when a device is to be used in a par- 
ticular application, it is often desirable to obtain 
directly information bearing on its efficacy in carry- 
ing out an assigned task under the conditions which 
prevail in the particular application. Such a test is re- 
ferred to as an operational test. 

The difference in philosophy of the two types of 
tests is essentially the following: A calibration test is 
made under carefully controlled conditions, with the 
object of eliminating all extraneous factors entering 
into the measurement which represent characteristics 
of the environment rather than those of the device it- 
self. In an operational test, on the other hand, en- 
vironmental factors are of prime importance, since 
information is desired not on the intrinsic character- 
istics of the device but on its behavior in an environ- 
ment closely approximating actual operating condi- 
tions. There is, of course, a relationship between the 
operational performance of a device and its inherent 
characteristics as determined by calibration measure- 
ments. Operational tests in general are beyond the 
scope of the activities of the Underwater Sound Ref- 
erence Laboratories and the present discussion is 
largely limited, therefore, to testing technique in 
calibration measurements. 

5.1.2 Yhe Characterization of Transducers 

Most calibration measurements on underwater 
sound equipment consist of measurements on trans- 
ducers, so that principal interest is attached to these, 
although much of the discussion is applicable to 


measurements on domes, baffles, and similar auxiliary 
equipment. An important consideration in the cali- 
bration of a transducer is the information which is 
required to characterize the device. A linear, passive, 
electroacoustic transducer*^ is completely character- 
ized when certain parameters and parametric func- 
tions are known as functions of frequency, as is shown 
in Chapter 8. When these relations are known, one 
can in principle compute the behavior of the device 
in any well-defined environment. However, neither 
the determination of the characteristic quantities 
nor the determination from these of the behavior of 
the instrument in even relatively simple environ- 
ments can actually be carried through because of the 
complexities of the measurements and computations 
necessary. Fortunately, however, such a complete 
characterization is neither necessary nor desirable 
under most circumstances. In the majority of cases, 
most of the useful information about a transducer 
can be obtained by relatively simple procedures, and 
operational characteristics can be derived from these 
data in a relatively direct and simple manner. 

The characterization of a transducer, as dictated by 
practical considerations, is summarized in Chapter 4. 
Therein are indicated the principal functional rela- 
tionships whose measurement gives information 
which, if not complete, is at least sufficient to charac- 
terize a transducer and to allow its operational be- 
havior to be evaluated for most cases of interest. 

The intent of the present chapter is to indicate the 
means by which one may determine the true values of 
the quantities measured; in other words, it is to find 
the means by which the measured values may be cor- 
rected to make the results independent of the charac- 
teristics of the equipment, the location of the tests, 
etc. This problem presents two aspects: (1) to deter- 
mine the conditions of the test so that local extra- 
neous factors do not enter significantly into the 
measurements and, (2) where the above is not pos- 
sible under the conditions present, to correct for the 
effects of local extraneous factors. 


a The definitions of these terms are found in Chapter 3. 


34 


THE TESTING PROBLEM IN GENERAL 


35 


The characterization of a transducer for practical 
purposes is usually effected by evaluating the follow- 
ing quantities: (1) receiving response as a hydro- 
phone, (2) transmitting response as a transducer, (3) 
directivity pattern, and (4) impedance. 

Receiving Response as a Hydrophone 

The receiving response as a hydrophone is dehned 
as the open-circuit voltage (in db vs 1 volt) generated 
by the hydrophone when placed in a uniform plane- 
wave sound field of reference pressure (1 dyne per sq 
cm) propagating parallel to the acoustic axis of the 
hydrophone. (See Chapter 4.) The acoustic axis of the 
hydrophone is an arbitrarily selected axis through the 
hydrophone, which is, however, usually chosen to be 
either some axis of symmetry for the instrument, the 
axis of maximum response, or some other readily 
identified axis. The plane-wave sound field of 1 dyne 
per sq cm should be, of course, the sound field when 
the hydrophone is not present, since the latter will in 
general distort the field by diffraction. If the hydro- 
phone is linear, the voltage generated is proportional 
to the magnitude of the sound field, and the measure- 
ment may be made in a sound field of any magnitude 
and then reduced to its value for a reference field. In 
some cases, where it is not possible to measure the 
open-circuit voltage of a hydrophone, the voltage 
across some given impedance is measured. This volt- 
age may or may not be reduced to an open-circuit 
voltage, depending on the circumstances. 

Transmitting Response of a Transducer 

A transmitter of finite size can be shown theoreti- 
cally to produce a sound field such that the pressure 
along any axis of the transmitter, at sufficiently great 
distances from the transmitter, falls off directly as the 
distance from the transmitter. This region at suffi- 
ciently great distances is known as the inverse-square- 
law region, since the sound intensity falls off as the 
square of the distance. Close to the transmitter the 
sound field does not follow this law. Here a more 
complicated sound field distribution obtains, which 
depends upon the particular characteristics of the 
transducer. For most applications only the pressure 
produced in the inverse-square-law region is of im- 
portance. 

The relationship between pressure and distance is 
given by 



for points in the inverse-square-law region, where p is 
the pressure at a distance d from the transmitter on 
any axis through the transducer, and Cq is a constant 
which may have different values for different axes. 

The transmitting response of a transducer is de- 
fined as the pressure measured at a distance d in the 
inverse-square-law region on the acoustic axis of the 
transducer for 1 watt available power from a given 
generator impedance (assumed to be purely resistive), 
reduced to a distance of 1 meter by multiplication by 
d in meters, and expressed in db vs 1 dyne per sq cm. 
(See Chapter 4.) The available power of a generator is 
defined as the power delivered by the generator to an 
impedance which is the complex conjugate of its own 
impedance. Thus, if the generator has a purely resis- 
tive impedance r^ and a generator voltage eg, the 
available power from the generator is given by 



(See Figure 1 in Chapter 4.) For a linear transducer 
the generated pressure is proportional to the current 
into the transducer (independent of its impedance). 
Therefore, to characterize a transmitter on the basis 
of available power, it is necessary to know not only 
the impedance of the source but the impedance of the 
transmitter as well. 

Directivity Pattern 

The trainsmitting and receiving responses charac- 
terize the acoustic behavior of a transducer at long 
distances on its acoustic axis; however, information is 
also desired on the sound field produced in other 
directions. This information is provided by directiv- 
ity patterns, which give the ratio in db of the response 
in any direction to the response on the axis. To char- 
acterize directions, it is necessary to set up a spherical 
coordinate system for the transducer. Referring to 
Figure 1, we take the Z axis along the acoustic axis of 
the transducer and choose the XZ plane in an arbi- 
trary orientation which is usually selected to be the 
horizontal plane through the transducer in its normal 
operating position. The direction of any line can 
then be specified by the angle 9 between the line and 
the Z axis, and the angle between the XZ plane 
and the plane through the Z axis and the line, as 
shown. To specify completely the directional pattern, 
the ratio of the response on every axis to that on the 
acoustic axis should be given. If the sound field is 


36 


TESTING TECHNIQUE 


rotationally symmetrical about the Z axis, however, 
only the pattern for lines in one plane (the XZ plane, 
for example) need be given. AVhen this symmetry does 
not exist, measurements of the patterns in a few 
planes are usually sufficient for practical applications. 

It may be noted from the definition that the direc- 
tivity pattern may be obtained for both hydrophones 
and transmitters. For linear passive transducers which 
obey the reciprocity principle, the directivity pat- 
terns for the transducer in receiving and in transmit- 
ting are identical (see Xo. 4 in Section 5.5.7). 

Impedance 

The iinpedance of a transducer is defined as the 
complex (vector) ratio of the voltage across the ter- 
minals to the current into the transducer. (See Chap- 
ter 4.) The impedance depends upon the acoustic 
termination of the transducer, and in principle the 
impedance should be measured when the transducer 
is in an infinite medium— water, in pai ticular. 

5.1.3 Ideal Testing Conditions 

An examination of the preceding definitions indi- 
cates that, in order to measure properly the quantities 
discussed in strict accordance with their definitions, 
one must have (1) an infinite homogeneous medium 
in which to perform the tests, (2) a source of plane 
waves, and (3) no extraneous acoustic signals (am- 
bient noise). AVhile these ideal testing conditions are 
impossible to meet in actual practice, they provide, 
nevertheless, a useful standard with which to gauge 
and appraise actual testing conditions. 

Compromises in Actual Testing 
Conditions 

Testing Media 

It is evident that no infinite homogeneous testing 
medium actually exists. If available bodies of water 
are considered, two departures from ideal conditions 
occur. Actual bodies of water are bounded by an air- 
water interface at the surface and by bottoms ranging 
in character from soft mud to rock. The influence of 
these boundaries on a calibration test lies in the fact 
that they reflect sound from the source so that the 
acoustic signal reaching a measuring instrument con- 
sists of the vector sum of the desired acoustic signal 
and these reflections. Dep)endingon the relative phase 
and magnitude of these reflections, the measured 


Y 



FiGiTtE 1. Coordinate svsiem for transducer. 


signal magnitude may be greater or less than the 
acoustic signal which one desires to measure. 

The obvious method of eliminating these effects 
would be to choose a site in which bounding surfaces 
are so distant from the testing locale that reflected 
signals would be negligible compared to the desired 
signal. Although this is a feasible compromise, there 
are limits dictated by other circumstances as to how 
far one can go in this direction. One of these, repre- 
senting the second departure from ideal conditions in 
actual media, is the lack of acoustical homogeneity of 
available bodies of water over large distances. These 
inhomogeneities are most often due to temperature 
gradients in the medium, although other effects, such 
as variations of salinity and increase of hydrostatic 
pressure with depth, are sometimes significant. The 
effect of these is to cause refraction of the acoustic 
signal, thus disturbing the desired acoustic geometry' 
of the test and in general introducing unknown fac- 
tors which influence the measurements. 

Plane IVaves 

The only means by which one can generate truly 
plane waves in an infinite medium is by means of an 
infinite plane radiator. However, at sufficiently large 
distances in a homogeneous medium, all sound gen- 
erators of finite size produce waves for which the sur- 
faces of constant phase are concentric spheres. If the 
amplitude of the waves does not change appreciably 
over the volume occupied by the device being tested, 
and if the radius of the spheres of constant phase at 
the test fxjsition is sufficiently large compared to the 
dimensions of the device, then for all practical pur- 
poses the wave field in the region may be considered 
as plane. The greater the testing distance, the more 


SELECTION OE TESTING SITE 


37 


nearly is this the case. The testing distance is limited, 
however, by the interfering reflections which arise at 
the bounding surfaces of medium. A compromise be- 
tween these two factors must be effected, and much of 
the later discussion centers around this point. 

Ambient Noise 

To insure that only the desired signal is being 
measured, it is necessary that the ambient noise, 
which is invariably present in any body of water, be 
of sufficiently low level compared to the level of the 
desired signal. In this matter, the best that may 
usually be achieved is the selection of a site where 
ambient noise is naturally low. When calibrating re- 
ceivers, a further gain may be realized by using trans- 
mitters which can deliver an acoustic signal of 
sufficiently high level to make the effect of ambient 
noise of negligible importance. 

Necessity for Testing Technique 

The preceding discussion indicates that the specific 
manner in which a test is conducted is usually a com- 
promise among several competing extraneous effects 
which tend to prevent a measurement under ideal 
testing conditions. The purpose of a doctrine of test- 
ing technique is to provide a body of rules to aid in 
determining the most advantageous compromise in 
the choice of such factors as testing site, depth, and 
distance, and thus to permit the most accurate meas- 
urement possible of the quantities desired. These 
rules are based partly on theoretical considerations 
and partly on general experience in such measure- 
ments. They may play as important a part in deter- 
mining the accuracy of the results as the quality of 
the testing equipment available. In fact, a lack of ap- 
preciation for these rules often introduces far greater 
errors in a measurement than any other factor. 

52 SELECTION OF TESTING SITE 
Available Sites 

The first question which arises in a program of 
calibration testing is the choice of a site for the meas- 
urements. Available sites ysually fall into the follow- 
ing five groups: 

1 . Oceans and large lakes. These may be character- 
ized as bodies of water many miles in extent and 
more than 100 feet in depth. They are of sufficient 
size so that, except in relatively calm weather, surface 


waves of more than 1 foot in height are usually pres- 
ent. To reach great depths one must usually go at 
least several thousand feet out from shore. 

2. Small lakes. Inland bodies of water ranging 
from 200 yards to several miles across may be classi- 
fied as small lakes. Their depths may vary greatly but 
rarely exceed 100 feet. Surface waves are relatively 
small and, in many cases, considerable depth may be 
reached within 50 to 100 feet from shore. 

3. Natural or artificial ponds. Bodies of water up 
to about 200 yards across, with various depths usually 
less than 50 feet and with relatively small surface 
waves, are classified as natural or artificial ponds. 

4. Rivers. Although rivers vary greatly in char- 
acter, they have more or less steady water currents. 
They have depths rarely exceeding 50 feet and, if 
navigable, usually carry considerable water traffic. 

5. Tanks. Tanks are usually internally housed 
structures of relatively small dimensions. For pur- 
poses of further discussion, swimming pools may be 
considered as tanks. 

5.2.2 Factors Entering Into Choice of Site 

Certain factors entering into a choice of testing site 
in relation to compromises in actual testing have 
been previously discussed, and are now considered in 
detail along with other factors. 

Size 

The factor of size enters into the choice of site in 
two ways: in its direct effect on measurements caused 
by reflections from the surface, shore, bottom, etc., 
and indirectly in its determination of other factors 
such as homogeneity of the medium, ambient noise 
level, accessibility, and rigging considerations. If 
these latter considerations were not important, the 
largest and deepest body of water available would be 
the most preferred, for it would then approach most 
closely to an infinite medium, and the effect of in- 
terfering reflections could be essentially eliminated. 
Actually, however, a compromise must be effected 
among these various factors. 

Type of Bottom 

The surface of a body of water being in almost all 
cases an air-water interface which reflects sound al- 
most completely, there is not much choice in this con- 
sideration. However, the type of bottom which is 
present may often be of importance. Types of bottom 


38 


TESTING TECHNIQUE 


available may be classified as mud, sand, and rock. 
1 he last is the least desirable, since it is usually a good 
reflector. A soft mud bottom which does not contain 
gas bubbles is probably the most desirable of the 
three. However, most soft mud bottoms contain or- 
ganic material whose decay produces bubbles of gas. 
Part of this gas remains entrapped in the mud and 
makes it a relatively good reflector. 

A fine sand or silt may be fairly absorbing. It has 
been found, for example, that the fine sand bottom 
at Lake Gem Mary in Orlando is a better absorber 
for sound than the soft mud bottom at Crystal Lake 
in Mountain Lakes. The inferiority of the latter has 
been traced to the presence of gas bubbles in the 
bottom. No satisfactory method has been found to 
inhibit permanently the decay which produces the 
bubbles, but repeated dredgings can keep the bottom 
in a satisfactory state. 

Transmission and bottom reverberation measure- 
ments at sea also have indicated that rock, sand, and 
mud are successively better absorbers, in the order of 
listing. 

Ambient Noise 

The ambient noise present in the water at a testing 
site determines the minimum signal pressure which 
can be measured at the locations. Ambient noise 
is usually not a significant factor in measurements 
except when the transmitters used have a sound out- 
put limited to low values. Ambient noise also deter- 
mines the lower limit at which inherent self-noise 
measurements on transducers may be made. 

The ambient noise at a testing site may be due to 
many factors. It may be produced by waves on the 
surface or lapping against the shore (particularly in 
rough weather), underwater life, water traffic, air- 
borne sound, rain, or various other sources. Hence, 
to keep the ambient noise as low as possible, one 
should choose a location where waves are relatively 
small, no water traffic is present, and noise-producing 
underwater life is absent. Small lakes, ponds, or tanks 
meet these conditions best. 

Accessibility and Weather 

An obvious but important consideration in choos- 
ing a testing site is the accessibility of the site itself. 
If one goes to a large lake or ocean and testing is done 
far from shore, the matter of transportation to the 
testing site becomes important. Furthermore, testing 
may be made impossible at such a site in any but fair 


weather so that the time available during the year 
for testing may be relatively short. In small lakes, 
ponds, or tanks, this is much less of a problem, and it 
has been found that on small lakes testing may pro- 
ceed satisfactorily except during heavy storms. A 
homely but significant factor, if testing is done from 
ships or barges in large bodies of water, is the possi- 
bility of seasickness among the testing crew in any 
sort of weather where pitching or rolling of the vessel 
occurs. 

Temperature Gradients 

Temperature gradients in water are usually of the 
same order of magnitude, regardless of the size of the 
body of water. The general effect of temperature 
gradients is the refraction of the sound beam. The 
most effective remedy is to work at testing distances 
as short as possible. Consequently a location where 
other factors allow a short testing distance is in gen- 
eral more satisfactory from the point of view of tem- 
perature gradients. Climatic conditions conducive 
to minimizing temperature gradients in water are to 
be preferred. These include cloudy days, rough water, 
or ice on the surface. The last two interfere, of course, 
with other aspects of calibration. 

Rigging Considerations 

By rigging is meant the general mechanics of hold- 
ing transducers and auxiliary equipment in a particu- 
lar location and with particular orientation. The 
testing site has some influence on the problems of 
rigging. For many tests, it is necessary to maintain 
instruments at a constant distance apart and to main- 
tain constant their relative orientation. This problem 
is simplified if the rigging is done from a rigid struc- 
ture. Therefore, piers are more satisfactory testing 
locations than ships or barges, so far as ease of rigging 
is concerned. When rigging is done from a ship or 
barge, the supporting structures require additional 
rigidity because of pitching and rolling of the vessel. 
In addition, a pier allows vertical hanging of an in- 
strument with comparative ease. The greater the 
testing depth, the more difficult it is to provide a 
rigid structure to maintain instruments at their loca- 
tions. Thus, the advantagQ^ of working at great depth 
to avoid surface reflections are partly vitiated by the 
additional difficulties in rigging. 

Location of Test Equipment 

In the performance of calibration tests, there is 


SELECTION OF TESTING SITE 


39 


required, in addition to the transducers used in the 
tests, a considerable amount of auxiliary electrical 
equipment such as amplifiers, modulators, imped- 
ance bridges, and recorders. This equipment should 
all be located in relative proximity to the transducers 
which are used, so that transmission losses occurring 
in the propagation of electrical signals from trans- 
ducers to measuring instruments are as small as pos- 
sible. If possible, it is most satisfactory to have the 
electrical measuring equipment directly over the 
testing location, although signals may be transmitted 
from the end of a short pier without serious loss. 
Transmission of signals from a barge to shore, how- 
ever, may not be as satisfactory as is to be desired. 
Thus, the location of the test equipment should be 
taken into account in selecting a test site. 

^•2-^ Small Lakes as Testing Sites 

In Sections 5.2.3, 5.2.4, and 5.2.5, the relative ad- 
vantages and disadvantages of the various available 
testing sites are discussed. USRL has largely con- 
fined its testing to small lakes and indoor tanks. The 
former have been found to be a relatively satisfactory 
type of site for calibration testing. WTile they have 
certain disadvantages, the methods developed to 
compensate for these have proved quite successful. 

The principal disadvantage of a small lake is its 
relatively shallow depth and, in some cases, its small 
size. This usually limits the testing distance that can 
be used, principally because of reflections from the 
surface and bottom. However, methods described 
later in this chapter have made it possible to elimi- 
nate, to a considerable extent, the effect of these re- 
flections on tests, except at relatively low frequencies 
(principally below 10 kc). On the other hand, a small 
lake has the advantages of low ambient noise level 
and the possibility of working in all but very bad 
weather. 

If testing is done from a pier, as is usually the case 
at Mountain Lakes and Orlando, the problem of 
rigging for tests is fairly easily solved, and equipment 
may be located on shore without introducing serious 
transmission line problems. Small lakes are also usu- 
ally free from rapid currents and tides, thus simplify- 
ing the problem of rigging even more. While 
temperature gradients of sizable magnitude occur in 
small lakes, the small testing distances which are 
used (rarely over 50 feet) reduce the effect of this 
factor to relative insignificance.’’ A general disadvan- 


tage of all outdoor testing sites is the fact that the 
temperature of the water cannot be controlled, as it is 
determined by climatic and weather conditions. 

In general, the same considerations apply to ponds 
as to small lakes, but the relatively smaller size in- 
troduces the difficulty that reflections from the shore 
may be quite serious, especially in such tests as the 
rear response of highly directional projectors. On the 
whole, however, small lakes probably provide the 
most generally satisfactory outdoor testing sites for 
calibration testing.^^ 

Large Bodies of Water and Rivers 
as Testing Sites 

The principal advantage of a large body of water, 
such as the ocean or a large lake, is the great depth 
which usually can be attained. This reduces interfer- 
ing reflections and permits the use of greater testing 
distances, thereby eliminating proximity effects in 
calibration. 

The number of disadvantages is quite large, how- 
ever. The ambient noise level is generally high be- 
cause of large waves and water traffic. Accessibility 
becomes a serious problem and is usually severely 
limited by weather conditions. The rigging of test 
devices at sufficient depths to realize the advantage 
of deep water is a major problem. Long rigid suspen- 
sions must be employed and these must be of suffi- 
cient strength to withstand the bending moments 
induced by rolling and pitching of the vessel from 
which the instruments are suspended. Large vessels 
must be employed if the greater testing distances are 
to be attained, while if two separate vessels are used 
to support the receiver and transmitter, the problem 
of maintaining relative orientations becomes exceed- 
ingly serious. The effect of thermal gradients also 
becomes important. On the whole, it may be said that 
such difficulties make it inadvisable to employ large 
bodies of water as test sites for calibration work, 
although they are essential to operational testing. 

Rivers suffer many of the disadvantages of oceans 
or large lakes, such as a high ambient noise level due 
largely to water traffic, frequent periods of inacces- 

Other laboratories have sometimes indicated considerable 
trouble caused by temperature gradients, but this has not been 
the experience at the USRL test stations. 

c Experience has indicated that there is no appreciable dif- 
ference in the behavior of underwater sound equipment in fresh 
water as compared to sea (salt) water. 


40 


TESTING TECHNIQUE 


sibility, and difl&culties in satisfactory rigging, which 
are increased by water currents. In general, the ad- 
vantages of a river, without the disadvantages, may 
be realized just as satisfactorily in a lake of proper 
size. 

5.23 The Use of Acoustic Tanks 
for Testing 

A disadvantage presented by all natural testing 
sites is that the temperature of the water cannot be 
controlled. The hydrostatic pressure can be con- 
trolled somewhat by the choice of testing depth but 
is limited by the depth of water at the location and by 
other factors, such as rigging problems. In the cali- 
bration of many devices, the determination of the 
temp>erature and pressure dependance of certain of 
their characteristics is both desirable and necessary, 
since many of these devices must be operated in water 
temf>eratures ranging from near-freezing to tropical 
and at hydrostatic pressures corresponding to depths 
of several hundred feet of water. The only satisfactory 
method of making such measurements seems to be 
by the use of an acoustic tank. The principal dis- 
advantage of tanks is their small size, which causes 
interfering reflections and limited testing distances. 
However, if the inner surfaces of the tank can be 
coated with some sound-absorbing material, or if a 
pulsing technique in measurement can be employed, 
a tank may form a satisfactory testing site. It has the 
advantage of having low ambient noise, accessibility 
independent of weather, and simplified rigging, in 
addition to temp>erature and pressure control. The 
characteristics of particular tanks that have been con- 
structed are treated elsewhere in this volume. 

5^ ELIMINATION OF REFLECTIONS 
General Considerations 

It was pointed out previously that acoustic reflec- 
tions from the surface, bottom, or shores of a body 
of water, or from other reflecting objects such as pil- 
ings used to supjx)rt a pier or dock from which tests 
are made, interfere with the calibration of instru- 
ments since they make it difi&cult to establish a plane 
progressive sound wave. Receivers in such cases meas- 
ure not only the pressure or pressure gradient due to 
the desired wave from the source, but also the contri- 
butions of reflected waves to the pressure or pressure 


gradient. The superposition of the direct wave and 
reflected waves produces in the medium a compli- 
cated standing wave pattern, whose configuration 
changes as the frequency is varied. Thus, if a nondi- 
rectional (pressure-sensitive) receiver is used to meas- 
ure the pressure at a point in the resultant field, the 
measured pressure oscillates about the pressure in 
the direct wave as the frequency is varied. Assuming 
the surface to be perfectly reflecting, which is very 
closely true in many tests, the expression for the pres- 
sure at a horizontal distance d from a point source 
at a depth h and with an operating frequency / is 


P = [po" 


+ 


Po^d-^ 


-f 4/z2 


where po is the pressure which would exist at the 
point in question if the water surface were not pres- 
ent, and c is the velocity of sound. Thus, as f varies, p 
oscillates between the values 

fob + 


because of the cosine term. It will be noted that the 
oscillation of p with / becomes more violent as d be- 
comes greater compared to h. These phenomena are 
well illustrated in Figure 2, where the voltage devel- 
oped by an essentially nondirectional hydrophone 
in the field produced by an essentially nondirectional 
source is shown for several testing distances. For any 
fixed depth the interference minima are increasingly 
prominent as the testing distance is increased. The 
characteristic appearance of these interference maxi- 
ma and minima is quite helpful in indicating when 
reflections are interfering in a test. 

If the reflecting surface, which may be the bottom 
or any other surface, has a pressure reflection coeffi- 
cient R, then equation (3) is modified to 


\po- + 


2R^Po- 

(d^ -f 4/z2)^ 


R‘-po-d 
d^ + 4/z2 


\^Vd^TW^-d)~a\J 


( 5 ) 


where a is the phase shift on reflection. Hence, by re- 
ducing the reflection coefficient of the surface, one 


ELIMINATION OF REFLECTIONS 


41 


may reduce the magnitude of the response variation. 
An air-water interface has a reflection coefficient close 
to unity, but a muddy or sandy bottom may have a 
considerably smaller one. It is evident that R may 
also include any other factor which makes the re- 
flected wave have a smaller amplitude, such as direc- 
tionality of the source or of the receiver. 

It may also be noted that the frequency spacing 
A/ between successive maxima or minima is given by 

A/ = ^ (6) 


where aL is the difference in path length of the 
direct and reflected waves, its value in the above case 
being ( Vd- + 4/i2 - d) • 

The partial elimination of reflection interference 
in test results can be accomplished by (1) reducing 
the effective value of R by the use of directional 
sources, screens, etc., (2) averaging out the interfer- 
ence effects through the use of warbled frequency or 
other multi-frequency signals, (3) making use of 
pulses which are measured before reflected pulses 
reach the point of observation, and (4) mathemat- 
ically eliminating the effects of reflection from the 
results. Each of these methods has some advantages 
and some disadvantages which are discussed in de- 
tail in the sections following. 

Directional Sources 

The simplest and one of the most effective methods 
of eliminating any significant reflections in calibra- 
tion tests is by the use of directional sources. These 
must be so designed and oriented that very little 
acoustic energ)’ reaches the surface or bottom in the 
direction in which direct specular reflection from 
the surface or bottom to the position of the receiver 
occurs. Sources with various types of directivity pat- 
terns may be employed, but, for practical considera- 
tions, only three require attention: the dipole source, 
the circular piston source, and the line source. 

If the reflecting surface is assumed to be a perfect 
reflector, and if the directivity pattern of the source 
is given by I(6)/Iq where I (6) is the intensity pro- 
duced at a given distance in a direction making an 
angle 6 with the axis of the source (assumed to be 
along the line joining the source and receiver) and Iq 
is the intensity produced at the same distance on the 
axis, then R- in equation (5) may be put equal to 



FREQUENCY IN KILOCYCLES PER SECOND 


Figure 2. Effect of testing distance on response of 3-A-32 
pressure hydrophone measured with 1 J-4 projector. Test- 
ing depth = 9 ft. Water depth = 14 ft. Testing distance 
as shown on cur\es. 


I{dr)/Io where Or is the angle indicated 
Now Or is given by 

in Figure 3. 

Or = tan-i^, 
a 

(7) 

so that 


i?2 = /(tan->^)/o. 

(8) 

For a dipole source with axis along the line joining 
source and receiver 

lo 

(9) 

so that (using subscripts to indicate 
source). 

the type of 

R^- = cos2(tan-i ^) 


(R , 4/i2 

+ 4/i2 d'l + 4/i2' 

(10) 


For a circular piston with axis along the line joining 
source and receiver. 



A 


42 


TESTING TECHNIQUE 



Figure 3. Geometry of reflection interference in calibra- 
tion tests. 


with the result that 



so that 


R 


2h y 

\ A ‘ \/d^ + 4/? V 
ttL 2h 

A * \/d^ + 4/72 


(14) 


Since the side lobes for a circular piston and for a 
line are of minor importance in the consideration of 
reflections, R^i, Rp, and Ri may be given approxi- 
mately (provided hjd is not too large) by the expres- 
sions 



(10) 


(15) 


(16) 


2a( 


ttD 

A 


2h 


-h 


ttZ) 

~k 


2h 


Vd2 + 4/72 


( 12 ) 


The term D is the diameter of the piston, A the wave 
length, and Ji(x) the Bessel function of unit order. 
For a line source of length L suspended vertically. 


m 

lo 


(fsiue) 


ttL 


(13) 


The reflected intensity reaching the receiver versus 
the intensity of the direct wave is then given in db by 


10 


r ( 

1 a w 




(17) 


where a = 1 for a pressure-gradient receiver, a = 
^4(7rD/A)2 for a circular piston, and a = ^(TrL/A)^ 
for a line. This equation is plotted in Figure 4. It is 
seen that, for reducing the effect of reflections, a 
piston is more satisfactory than a line whose length is 
equal to the diameter of the piston. At low fre- 
quencies, however, where A becomes large, both a line 



Figure 4. Surface reflected intensity versus direct intensity for directional sources, h = depth, d = testing distance. 
For dipole source « = 1 ; for circular piston source « = D = diameter of piston, \ = wavelength; for line 

source suspended vertically, « == 1/3 L = length of line. 


ELIMINATION OF REFLECTIONS 


43 




Figure 5. Effect of orientation of line source on magni- 
tude of surface reflections. 


and a piston become ineffective, but a dipole, whose 
pattern is independent of the frequency, retains its 
effectiveness. However, the dipole is inferior to a 
piston or a line at high frequencies. 

At high frequencies, a piston is the most effective 
source for eliminating interfering reflections. How- 
ever, before final conclusions as to its usefulness can 
be drawn, account must be taken of the fact that at 
relatively close distances the sound field of a piston, 
even in the absence of reflecting surfaces, is not a free 
progressive plane wave. This limits the minimum 
value of the testing distance that may be used. Simi- 
lar considerations apply to other sources. A discus- 
sion of the choice of both testing distance and source 
will be taken up in Section 5.4 when reflections and 
so-called proximity effects will be considered. 

Another fact should be pointed out. ft is difficult 
to construct a dipole source which develops an ap- 
preciable radiation field. This is a result of the fact 
that the radiation impedance of a dipole source is 
largely reactive, so that the pressure and particle 
velocity in the field are almost in quadrature at short 
distances from the source, and therefore little energy 
is radiated. 

Finally, the characteristics of the receiver must be 
weighed in considering the elimination of reflections. 
It has been assumed that the receiver in the preceding 


discussion was essentially a pressure-sensitive device. 
Some receivers are essentially velocity or pressure- 
gradient actuated. The problem is somewhat simpli- 
fied here, since the velocity components of the re- 
flected wave are not parallel to the axis of the receiver 
when it is oriented toward the source. Such receivers 
thus discriminate against surface reflections. How- 
ever, most receivers have more complex behavior and 
their discrimination against surface reflections can be 
judged by their directional patterns. Proximity ef- 
fects in the receiver are also a consideration limiting 
the shortness of testing distances. Thus the effective- 
ness of directional sources in discriminating against 
surface reflections can be properly judged only when 
considered with other limitations on testing distances. 
At this point one can only point out the potential 
value of directional sources in testing. 

5.3.3 The Choice of Instrument 

Orientation 

The preceding discussion regarding the use of di- 
rectional sources to reduce reflections suggests that 
the choice of orientation of the instruments being 
tested may make a difference in the prominence of 
reflection interference. When the instruments have 
their own directional characteristics, these often can 
be used to advantage. One prominent example is 
shown in measuring the frecpiency response of a line. 
The directionality of a line hydrophone is such that 
most of the acoustic energy is contained in the region 
between two cones having a common vertex and axis, 
the latter coincident with the line itself. The pattern 
is roughly like a pancake. If the line is suspended 
horizontally, the acoustic power incident on the 
hydrophone from the point on the surface from 
which reflections are received generates nearly as 
much voltage as the acoustic energy coming directly 
from the source (if the latter is nondirectional), as is 
shown in Figure 5 A. However, if the line is suspended 
vertically, this is no longer the case, for the hydro- 
phone is then relatively insensitive to signals coming 
from the surface. (See Figure 5B.) Thus considerable 
reduction in reflection interference can be effected 
by choosing the vertical orientation for the line 
rather than the horizontal one. Unfortunately, it is 
not usually possible to use the same scheme when 
directivity patterns for the line are being measured. 

A second important case occurs in connection with 
the measurement of the rear response of a projector 


44 


TESTING TECHNIQUE 


or hydrophone, when the receiving directivity pat- 
tern is being obtained. If the transmitter is a direc- 
tional one, it shonld be so oriented that the principal 
part of its acoustic energy is directed away from 
shore. If the projector is oriented in the opposite 
direction, a large part of the energy is reflected from 
shore and reaches the receiver coming from that 
direction. When the receiver is oriented to measure 
rear response, it is most sensitive to sound arriving 
from shore. In such a case, the reflected signal voltage 
may greatly exceed the direct signal voltage. With 
the other orientation this difficulty is avoided. This 
is shown quite clearly in Figure 6. 

Another scheme which often may be of value in 
calibrating a source is to use a velocity or pressure- 
gradient sensitive receiver. Such a receiver is theoret- 
ically insensitive to signals coming in from a direction 
at right angles to its axis. By orienting the source and 
receiver as shown in Figure 7, the receiver may be 
made almost entirely insensitive to reflected signals 
from the surface. The difficulties of rigging the in- 
struments in the proper orientations as shown have 
essentially prevented this method from being used by 
USRL. 

Use of Screens, Lenses, and Orifices 

Another method of eliminating reflections which 
readily suggests itself is the use of screens or baffles 
to shield the receiver from any but the direct waves 
from the source, much as is done in optics for similar 
reasons. This method is not as effective as first 
thought might indicate because the acoustic wave 
lengths in which one is interested are generally of 
the same order of magnitude as the dimensions of 
suitable baffles, unlike the corresponding situation 
in optics. Therefore, in the range of frequencies of 
usual interest, geometrical acoustics is a far from 
valid representation of acoustic phenomena. How- 
ever, in spite of the importance of diffraction in such 
cases, some useful results are obtainable by the proper 
use of screens or baffles. 

1 he collimation of light by lenses or pinholes is a 
well-known optical method for preventing stray re- 
flected light from interfering with a measurement. 
Would it then be possible to use the same methods in 
acoustics? Consider first the analogue of a pinhole, 
that is, a large screen in which an orifice is cut, and 
the source and receiver placed on opposite sides. (See 
Figure 8.) If the source is placed far enough behind 



Figure 6. Dependence of interference from shore reflec- 
tions on arrangement of instruments in measuring rear 
response. 


the screen so that the waves reaching the orifice are 
essentially plane, it is found by a calculation using 
the wave equation that the resultant sound field on 
the receiver side of the screen is the same as though 
the orifice were considered a piston source of the 
same diameter. The same considerations apply to 
such orifices as to directional sources of the piston 
type which have been discussed in the previous sec- 
tion. One is again limited by proximity effects if the 
receiver is put too close to the orifice. On the other 
hand, the beam through the orifice has a finite angu- 
lar divergence and strikes the water surface and is 



Figure 7. Orieutatiou of pressure-gradient receiver to 
eliminate interference from surface reflections. 


ELIMINATION OF REFLECTIONS 


45 



Figure 8 . Use of screen with orifice to reduce surface and 
bottom reflections in calibration. 


reflected to the receiver again, if the latter is placed 
too far away. Finally, one cannot place the source too 
close to the orifice or else the transmitted beam will 
have too great an angular divergence and thus strike 
the surface near enough to give a reflected wave to the 
receiver. Therefore, an orifice in a screen can be con- 
sidered only as a method of producing an effective 
piston source of larger area than would be practical 
for the diaphragm of a projector. 

A similar analysis of the feasibility of using acous- 
tic lenses leads to the same conclusions: a lens also 
acts in principle like a piston source of the same 
diameter. The construction of suitable acoustic lenses 
involves various technical difficulties in addition to 
the theoretical ones outlined above. 

A helpful yet considerably simpler method for 
reducing reflections is by using, at the surface or 
bottom, baffles or screens so oriented as to cause re- 
flections to be directed away from the receiver. Such 
screens may be considered to reduce the effective 
value of the reflection coefficient R. While they may 
be placed so that, according to geometrical acoustics, 
no reflected sound should reach the receiver, diffrac- 
tion about the screen and waves on the water surface 
usually allow still a considerable part of the reflected 
sound to reach the receiver. For a given size reflector, 
the diffraction is greater at lower frecpiencies. This is 
unfortunate since the low-frequency region is just 
the region in which directional projectors also be- 
come difficult to construct. 

The type of screen used by USRL is essentially a 
watertight sandwich of i^-inch hard green felt en- 
closed between y^.^mch galvanized iron sheets, and 


having dimensions roughly 2x4 feet. The layer of 
enclosed air acts as an effective reflector, and the felt 
is intended to damp out resonant frequencies.^" 

Two arrangements of screens have been employed. 
One consists of hanging the screen vertically with the 
top edge just breaking the surface midway between 
the source and receiver, with the plane of the screen 
perpendicular to the line joining the two instru- 
ments. (See Figure 9 A.) A more effective arrange- 
ment has been found to be a pair of screens in the 
form of a “V” suspended at the surface in the position 
shown in Figure 9B. fn this arrangement, sound re- 
flected from the screen is thrown off more or less side- 
wise along the surface. A series of such screens placed 
end to end along the entire distance between source 
and receiver (Figure 9C) is still more effective. While 



C 

Figure 9. Use of screens to reduce surface and bottom 
reflections. 


46 


TESTING TECHNIQUE 


the effectiveness varies with frequency, at high fre- 
quencies, where diffraction is less important, such an 
arrangement of screens may reduce the effective re- 
flection coefficient i? by a factor of from to I/ 2 . 
The ease of construction and handling makes it prof- 
itable to have such screens available for use. They 
may also be used in an inverted “V” arrangement on 
the bottom to reduce bottom reflections. 

It would probably increase the effectiveness of a 
screen to make it absorbing rather than perfectly 
reflecting. However, until recently no suitable sound- 
absorbing materials for underwater use have been 
available, and it is not yet known how effective these 
may be. 

5.3.5 Electric Signal Methods: 

Thermal Noise, Warble 

There are several methods of reducing the effect 
of reflection interference which have been used with 
considerable success in air acoustics. These are based 
on the fact that the cosine term in equation (3) is a 
function of frequency, so that, if the response is 
averaged over a band of frequencies, the oscillatory 
effect of this term on the response, as the frequency is 
varied, can be largely averaged out. Two methods of 
doing this are by using a frequency which is warbled 
about the frequency at which the response is desired, 
and by using a band of thermal noise centered at the 
frequency at which the response is desired. The re- 
sponse as measured by each of these methods is an 
average over a band of frequencies of the response of 
the instrument. The fact that a band of frequencies 
rather than a single frequency is used limits the reso- 
lution in response of the instrument. Rapid changes 
in response become more gradual as measured by 
this method. 

It may be shown that a signal which is warbled be- 
tween frequencies /o — A//2 and fo + A// 2, with a 
warbling rate very much less than /o, has frequencies 
in its harmonic (Fourier) analysis covering essentially 
the same frequency range as does the warbling. Simi- 
larly, a band of noise of frequency breadth A/ cen- 
tered at /o also contains, by definition, frequencies in 
this same band. It can be shown mathematically that, 
to eliminate the effect of interference oscillations in 
response by either of these methods. A/ must be deter- 
mined by the inequality 


where AL is the shortest difference in path between 
the direct signal and any of the reflected signals.'^^^ 

The extent to which one is interested in resolving 
the frequency variations in response of an instrument 
depends to a large extent on the type of instrument 
being tested. The resolution attained in a method of 
measurement of the response is defined in the follow- 
ing manner: Consider an instrument which has two 
very sharp response peaks at frequencies / — A//2 
and / + A//2. A method of measurement which is 
just able to resolve the response into two maxima is 
said to have a resolving power at the frequency / 
equal to 

RP = A (19) 

Therefore, the higher the resolving power, the greater 
the definition with which the method can measure a 
frequency response. For the average testing program, 
it is usually satisfactory if the resolving power is equal 
to 100 or 200 at all frequencies. For the use of warble 
or noise, the resolving power is 

RP = -L< —L IMi. (20) 

A/ c 

AL 

Hence, for any fixed frequency, the resolving power 
is determined by AL. This limits seriously the use of 
the method at low frequencies, since, with AL fixed 
as it is by the geometry of the test, the resolving power 
decreases as the frequency is lowered. Thus for 
AL = 5 feet, the maximum resolving power becomes 
100 at 100 kc, 10 at 10 kc, 1 at 1 kc. For rough meas- 
urements 10 might be admissible, but any lower 
values for the resolving power would give very little 
information of importance. Thus, it is only at high 
frequencies that the method is of much value. 

These methods have another serious disadvantage. 
After the oscillatory interference terms are elimi- 
nated in the expression for the pressure, the pressure 
that is measured is approximately the square root of 
the sum of the squares of the pressures in the direct 
wave and in all of the reflected waves reaching the re- 
ceiver. Thus, the measured pressure is always greater 
than the pressure in the direct wave alone. In some 
cases, such as the measurement of directivity patterns, 
the reflected waves are often higher in level than the 
direct wave, so that in this case the direct wave is ac- 
tually discriminated against by these methods. There- 


ELIMINATION OF REFLECTIONS 


47 


fore they can be considered useful only in establish- 
ing the general shape of the frequency-response curve 
but not in establishing its absolute level. Even the 
shape may be in considerable error. The greatest care 
is therefore required in using these methods to be 
sure that one is actually measuring the quantity 
desired. 

5.3.6 Electric Signal Methods: Pulses 

A more satisfactory method of eliminating the ef- 
fects of reflected waves in measurements is by the use 
of pulses. Instead of a steady signal, a pulse corre- 
sponding to a sinusoidal signal of finite duration is 
emitted. This pulse reaches the receiver by the direct 
path before the reflected pulses arrive. If the response 
of the receiver can be measured in the interval before 
their arrival, their effect is entirely eliminated. Since 
the front of the earliest reflected pulse arrives at a 
time AL/c after the beginning of the direct pulse, 
the response measurement must be completed in a 
time Tfn < ALjc after the arrival of the beginning of 
the direct pulse, where AL is the smallest difference 
in path between a direct and reflected signal. It is 
noted immediately that the pulse method has an im- 
portant advantage over the warbled-signal and noise- 
band methods in that the reflected waves play no part 
in the measurement of response, whereas in the latter 
methods a composite sum of the direct and reflected 
signals is measured. 

The limitations of the pulse method are indicated 
by the resolving power of the method. The Fourier 
spectrum of a pulse of finite length At can be shown 
to contain, essentially, frequencies covering a band of 
width 

( 21 ) 

centered at the signal frequency of the pulse. The re- 
solving power thus becomes 

^ ^ fAt (22) 

which, it would appear, could be indefinitely in- 
creased by extending the duration of the pulse. This 
is illusory, however, since what is measured at a time 
Tjn < ALjc must be independent of how long the 
pulse continues after the period AT/c has elapsed. 
One would perhaps guess that the actual resolving 


power would be that corresponding to a pulse dura- 
tion of T,n or 

RP = fr„^t^. (23) 

That this is actually the case will be shown now from 
other considerations. 

When a sinusoidal signal is applied to any electri- 
cal circuit containing inductances or capacitances or 
both, steady values of the currents and voltages are 
not immediately attained. At first there are present, 
in addition to the steady-state voltages and currents, 
transient voltages and currents which gradually die 
out with time. The time required for the transient 
essentially to disappear is known as the time constant 
of the circuit. For simple circuits this time constant t 
is independent of frequency, but for more compli- 
cated ones this may not be the case. For example, for 
a capacity C and a resistance R in series or parallel 

T = RC. 

For an inductance L and a resistance R in series or 
parallel 

L 

R' 

For a resonant circuit containing an inductance L, a 
capacitance C, and a resistance R 

2L 

when 

1 R^ 

LC ^ 4L2' 


^ _ _L 

2L V4L2 LC 

when 

1 R2 
LC ^ 4L2’ 

For simple resonant circuits it is convenient to in- 
troduce a quantity Q = 27r/oE/i?, where /o is the reso- 
nant frequency. Q essentially measures the number of 
cycles at resonance required for the transient to die 


48 


TESTING TECHNIQUE 


out effectively. It is also equal to /o / Afo, where A/o is 
the difference between the two frequencies, one on 
each side of resonance, where the current falls to 
l/\/2 times its value at resonance. Thus, one may 
consider it a measure of how peaked the response 
curve is and, therefore, a measure of how rapidly the 
current response varies with frequency. For more 
complicated circuits one cannot define an unambigu- 
ous Q, but may often speak of an effective Q, valid for 
some frequency range, such that Q measures the 
number of cycles required for the transient to die out. 
Associated with it is an effective time constant for the 
circuit T which is approximately related to Q by the 
relation Q = tt/qt. 

A transducer has many similarities to an electric 
circuit, and its response to a suddenly impressed 
signal can be judged by an effective time constant, or 
Q, for the transducer. In order to measure the steady- 
state response of a transducer, one must therefore 
wait a time considerably greater than r after the 
initiation of the signal, say t„i. Its value must be at 
least that expressed by 

> TTT. (24) 

To delineate carefully the response peak of a reso- 
nant transducer, one must have a resolving power 
greater than the ratio of resonant frequency to the 
breadth of the resonance peak, that is 

RP>^ = Q.-‘^foT. (25) 

Thus, we see that if we set the resolving power equal 
to /oT„i, as we conjectured earlier should be the case, 
we again get the condition > ttt, which is in 
agreement with the result obtained from the con- 
sideration of time constants. 

It then follows from the condition 

RP ^ (26) 

that the resolving power is limited in the same way as 
for warbled frequency or noise band signals, thus 
showing that pulses are likewise ineffective at low 
frequencies. A general criterion for the usefulness of 
pulses in obtaining the response of a resonant trans- 
ducer is 

(«P)..x = ^>S = ^ (27) 

c n:,jQ 


or 


AL > ^ ^ 

A/o 


Qc 

fo 


(28) 


The problem of determining the time constant of 
a transducer is not an elementary one in itself. In 
many cases one can determine it a posteriori for a 
resonant device, using the resultant Q obtained from 
the response curve by a pulse method. If this Q is not 
less than 7r/oT,„, there will be considerable doubt 
that the resolving power is greater than the Q. By 
viewing the received pulses of the transducer on a 
cathode-ray oscilloscope, one can usually obtain a fair 
estimate of the time constant. It should be pointed 
out that a transducer, in contrast to an electric cir- 
cuit, has its time constant determined not only by its 
electrical and mechanical elements but also by its 
acoustic geometry. In order for the local sound field 
surrounding the transducer to build up to its steady- 
state value, one must allow sufficient time for the 
sound waves to pass from one part of the transducer 
to another and build up the characteristic diffraction 
pattern about the transducer. The time required for 
the sound field to reach its local steady-state value is 
of the order of the linear dimensions of the trans- 
ducer divided by the velocity of sound. For a long-line 
hydrophone and even for smaller instruments, the 
acoustic time constant may be longer than that due to 
the electrical and mechanical elements. In any case, 
no response measurement should be made before the 
acoustic pulse has enveloped the transducer. 

The pulsing technique is particularly valuable in 
measuring directivity patterns, where the reflected 
signals may be higher in level than the direct signal. 
This is a case where other available methods often 
fail (see conclusion of Section 5.3.5). 

Methods for producing and measuring pulses are 
discussed in detail in Chapter 6 along with practical 
considerations in employing the pulsing technique. 
The results of pulse measurements on transducers 
may be found in several reports.55 

The procedures and problems involved in using 
the pulsing technique vary with different types of 
transducers. Careful thought is required before the 
method is used in any particular case, and prelimi- 
nary measurements are often helpful in determining 
its applicability. The preceding discussion should 
serve as a guide rather than as a rule in making pulse 
measurements. 


ELIMINATION OF REFLECTIONS 


49 


Corrections for Reflections 


In many tests it is found that the methods of elimi- 
nating reflections so far described either are ineffec- 
tive or, because of the particular nature of the test, 
cannot be used. In such cases, one must correct the 
results for the reflections which may have been pres- 
ent during the test. The principal difficulty in mathe- 
matically eliminating the effects of reflection inter- 
ference is the decision as to whether variations in re- 
sponse are a consequence of reflections or are inher- 
ent characteristics of the instrument under test. Some 
useful aids in identifying reflection interferences are 
the following: 

1. If response measurements are made at different 
testing distances,* the difference in path length be- 
tween the direct and reflected waves is not the same. 
As a result, the interference maxima and minima in 
the different response curves appear in different posi- 
tions. Since variations in the inherent response char- 
acteristic of the instrument under test are not shifted 
by changing the testing distance, this forms a valuable 
criterion for identifying reflection interferences. 

2. It was pointed out in Section 5.3.1 that reflec- 
tion maxima or minima are spaced regularly with 
frequency. This spacing between successive maxima 
or minima is 



( 6 ) 


where c is the velocity of sound and AT is the differ- 
ence in path between direct and reflected waves. 
Thus, a periodicity of response maxima or minima 
with frequency is often indicative of reflection inter- 
ference and, with the geometry of the test known, one 
can calculate the frequency spacing by equation (6) to 
determine whether or not it coincides with the ob- 
served values. If there is more than one prominent 
reflection entering into the test, however, this criter- 
ion becomes difficult to apply, since there are maxima 
and minima introduced by interference between the 
direct waves and each reflected wave and between the 
various reflected waves themselves. The resultant ef- 
fect on the response is quite confusing and makes it 
difficult to determine unambiguously whether varia- 
tions are due to reflection interference or are charac- 
teristic of the instrument. 

3. In calibrating sound sources, two receivers with 
different directivity patterns may be used. In such 
cases, reflection interferences have different magni- 


tudes or positions in the two frequency-response 
curves. This fact is often an aid in deciding whether 
variations in response are due to reflections or are in- 
herent in the instrument. 

Once the reflection maxima and minima have been 
identified in a response curve, it is necessary to decide 
how to eliminate them and obtain the inherent re- 
sponse characteristic of the device under test. The 
most useful method at higher frequencies makes use 
of the fact that, if the reflection maxima and minima 
are prominent and the direct and reflected signals do 
not vary too rapidly with frequency, the maxima ap- 
pear at frequencies where the direct and reflected 
signals are in phase, and the minima where they are 
out of phase. Hence, at a maximum, one measures the 
sum of the signals in the direct and reflected waves, 
and at a minimum, their difference. If one takes the 
signal voltages at a maximum and at an adjacent 
minimum, the arithmetic mean of these two values 
gives approximately the correct value. 

Probably the best way to make use of this principle 
is the following: Find the difference in level in db 
between each maximum and its two adjacent minima. 
Plot each difference against a function <^(/) of the 
mean frequency between that at the maximum and 
that at the minimum. The points may be connected 
in a smooth curve. Let D(j) be the voltage generated 
by the hydrophone due to the direct acoustic signal at 
sound frequency / and R(j), the voltage from the re- 
flected signal. The measured value at a maximum is 
expressed by D(f) + R(j) and at a minimum by D(f) — 
R{f). If these are expressed in db from the usual basic 
level, the equation for the curve becomes 

</,(/) = 20 log [D(/) + R{f) ] - 20 log [D(/) - R(j) ] 


= 20 



m - m 

m + R{f) 


] 


= 20 log 


1 


1 


m 

D(f) 

m 

m 


(29) 


From this equation, R(f)/D(J) can be determined. 

To evaluate D(f), which is the quantity desired, 
make use of the identity 

20 logD(/) = 20 log[D(/)-f i?(/)] 

-201og[l+^]. (30) 


50 


TESTING TECHNIQUE 



by following the instructions given with the nomo- 
graphical chart, Figure 11. This method is useful for 
calibrating very low frequency projectors. 

In the calibration of receivers by the comparison 
method, the problem is somewhat simplified if the re- 
ceiver under test and the one serving as a reference 
standard are either both nbndirectional (pressure- 
activated) or both pressure gradient, since in this case 
the reference field, even though it contains reflec- 
tions, is the same for both instruments, and their rela- 
tive calibration is correct in spite of the presence of 
reflections.^^ 

5 4 CHOICE OF TESTING GEOMETRY 


Figure 10. Chart for obtaining correct level from differ- 
ence in level between interference maxima and minima. 


The first term is measured directly and the second 
calculated from the ratio determined above. The true 
level for the frequency at any maximum is then the 
measured value minus a determinable correction. By 
a similar procedure, the correction to be added to the 
measurement at a minimum is 


+ 20 log 


m 

D(fP 


(31) 


These corrections are most readily made from the 
curves of Figure 10 where the subtractive term from 
(30) and the additive (31) are plotted against <^(/). 
Thus, after constructing the maxima-minima differ- 
ence plot, one need only take the value of <^(/) corre- 
sponding to the frequency of each maximum and, on 
going to Figure 10, read the number of decibels to be 
subtracted from the maximum to give the correct 
level. A similar process gives the number of decibels 
which should be added to a minimum to find the cor- 
rect level. 

While the above method is very useful in many 
cases, it is not too satisfactory at low frequencies, since 
it is difficult then to locate unambiguously the maxi- 
ma and minima. There is no reasonably good method 
for correcting for reflections at low frequencies. One 
method may be of some aid, if only surface reflections 
are prominent and both the source and receiver are 
nondirectional. It is based on the fact that under 
these conditions one can actually calculate the ob- 
served pressure at the receiver in terms of the pressure 
which would be present if the reflecting surface were 
not present. This procedure can be carried through 


Depth 

The testing geometry for a calibration test refers to 
the testing depth and distance used. The optimum 
depth is usually determined by the testing site so that 
in general it remains the same during most tests. This 
depth is limited by the available water depth and, as 
a rule, its optimum value is that for which the magni- 
tude of reflections from the surface is equal to that of 
those from the bottom. If Rg and are the effective 
pressure reflection coefficients for the surface and the 
bottom respectively, hy, the depth of the water, h the 
testing depth, and d the testing distance, then the in- 
tensity of the reflected wave from the surface to the 
receiver is 


Is 


fts-hd- 
+ 4/1=’ 


(32) 


where /q is the intensity of the direct wave. The in- 
tensity of the reflected wave from the bottom is 


h 


d^ + 4 (h^-hY' 


(33) 


Placing these equal to express the desired condition 
and solving, gives 



For d small, this reduces to 

h ^ 1 

hw I + Ri) 

Rs 


( 35 ) 


CHOICE OF TESTING GEOMETRY 


51 



Figure 11. Correction chart for surface reflections at low frequencies for nondirectional instruments. 

Instructions: 

Let h = depth of hydrophone and projector. 
r = testing distance. 

X = wave length. 

p = RMS pressure at hydrophone with surface present. 

pg = RMS pressure at hydrophone if surface were not present. 

Calculate h/r and r/\. Proceed to the alignment chart on right and place a straight edge so that it crosses the scale 
marked h/r at the calculated value and the scale marked r/x also at the calculated value. Read the angle « at the 
intersection of the straight edge and the scale marked a. Proceed to the diagram on left. Find the circle with center 
at A whose indicated value corresponds to the calculated value of h/r. Find the point of intersection of this circle 
with the radial line corresponding to the value of a previously obtained. Then the value indicated on the circle, with 
center at B, on which this point lies will be the value of 

20 logi-. 

po 

Interpolate if necessary. 

Example. Let /i = 5 ft, r=: 10 ft, frequency equals 150 c. 

Then — = 0.5, 

r 


X = 


V 

1 


4800 

150 


= 32 ft in water. 


- = 0.313. 
\ 

From the alignment chart: « = 47°. 

P 

From the diagram: 20 log — 2.6 db. 

Po 



52 


TESTING TECHNIQUE 



Equation (34) is plotted in Figure 12 for convenient 
reference. In the above derivation it is assumed that 
and Rg are independent of the angle of incidence 
and thus of d and h. However, since these reflection 
coefficients do depend upon the angle of incidence, 
and since their effective values depend also on the 
directivity of the device being tested, equation (35) 
should serve as a guide rather than as a rule in choos- 
ing the testing depth. Usually a testing depth lying 
between 1/2 and ^ of the water depth is satisfactory. 
The more absorptive the bottom, the greater is the 
relative testing depth which may be used. 

Distance 


pendence of surface reflections on testing distance 
and depth has been discussed in considerable detail 
in preceding sections of this chapter. Now it is neces- 
sary to discuss the proximity effects in detail before a 
criterion for the selection of testing distance can be 
determined. Because of the reciprocity principle (see 
Chapter 3 and Section 5.5.6), proximity effects for a 
transducer are the same whether it is acting as a trans- 
mitter or as a receiver. 

^ Proximity Effect for 

Pressure-Gradient Receivers 

A pressure-gradient or velocity-type liNclrophone 
is one whose response is (at least over a certain fre- 
quency range) proportional either to the component 
of the pressure gradient or to the particle velocity 
of the sound field parallel to the axis of the hydro- 
phone, rather than to the pressure in the sound field. 
In a plane sound wave, the pressure gradient is pro- 
portional to the pressure in the sound field, the 
proportionality constant being independent of fre- 
quency. For spherical waves this is not the case 
except for sufficiently great distances from the center, 
where the wave front is essentially plane over the 
hydrophone. Since the calibration of pressure-gra- 
dient hydrophones usually is desired in terms of the 
equivalent plane wave pressure, it is then necessary 
to employ a spherical wave correction. 

To obtain this correction, one uses the equation 
for the pressure in a spherical wave at a distance r 
from the center 




(36) 


Choosing the optimum testing distance is even 
more difficult than choosing the optimum depth, 
since more considerations must enter into the deter- 
mination of the former. If the testing distance is too 
great, reflection interference becomes very promi- 
nent, and when a low intensity source is used, diffi- 
culties with ambient noise may arise. On the other 
hand, if the distance is too short, proximity effects 
due to the spherical wave front incident on the re- 
ceiver introduce errors into the calibration, or a 
standing wave diffraction pattern may be set up be- 
tween transmitter and receiver. Thus, the selection 
of the optimum distance must be made as a com- 
promise between these competing effects. The de- 


where is a constant, and k = 27r/A, A being the wave 
length. The radial component of the pressure gradi- 
ent is then given by 

( 37 ) 

The ratio of pressure gradient to pressure is therefore 
given by 

/ Po(l + jkr)e-j^r ^ 

\drj _\ r2 / _ (1 + /Qox 


CHOICE OF TESTING GEOMETRY 


53 


or its absolute value is 


dp 
dr 

T 

^V^here r is large and the wave front is essentially 
plane, this ratio becomes simply k. Therefore, if a 
pressure-gradient hydrophone, calibrated in terms 
of the equivalent plane-wave pressure, is placed in 
a spherical sound field at a distance d from its center 
and with its axis radial, it then indicates a pressure 
which is greater by the ratio of equation (39) to k, or 



Figure 13. Increase in sensitivity of pressure-gradient 
hydrophone in a spherical sound field. 




times the actual pressure present at its location. Note 
that d now replaces r in equation (39). Thus, the 
hydrophone indications should be corrected by this 
factor to obtain the true pressure. This correction 
factor in db is plotted in Figure 13 for four values of 
d. It is to be noted that it is most prominent at low 
frequencies. The correction factor is to be subtracted 
from the observed pressure in db to obtain the cor- 
rect value. 

One might conclude from the above that one may 
work at any testing distance with a pressure-gradient 
instrument and simply apply the above correction. 
However, it must be remembered that most trans- 
ducers do not have a spherical wave field in their 
immediate neighborhood. This is true in particular 
for piston-like transducers, where the sound field in 
the immediate neighborhood of the face of the trans- 
ducer is very complicated and does not become 
spherical for what is often a considerable distance 
from the diaphragm. Consequently, great caution 
must be used in applying this correction. 

5.4.4 Proximity Effect for Pistons: 

Axial Response 

Most acoustic transmitters and receivers are cou- 
pled to the acoustic medium by a diaphragm which 
oscillates in a direction normal to its plane under 
the influence of the pressure in the sound field when 
receiving, or under the electromechanical forces of a 
transducer when transmitting. The sound field of 
such a piston source of finite area falls off according 
to the inverse-square law at large distances, where 
the wave fronts are spherical. Close to the transducer. 


however, the sound field is very complex and cannot 
be considered spherical. Only the case of a piston 
situated in an infinite rigid baffle is amenable to 
simple theoretical analysis because then there are 
no edges to cause diffraction. The present discussion 
is limited to this case. The results may be applied 
satisfactorily to any piston whose dimensions are not 
small compared to a wave length. 

Consider first the case of a circular piston. The 
pressure along a line normal to the plane of the circle 
at its center is, at a distance r from the piston, 

p = 2pcva\m\j{'\/a^ + - r)\, (41) 


where p is the density of the medium, c is the sound 
velocity in the medium, t/q is the normal velocity of 
the piston, = 27r/A, and a is the radius of the pis- 
ton. This obviously does not vary inversely with r 
for small values of r. However, as r becomes large 
compared to a, the radical may be expanded to obtain 


p = 2pcv^^ 



(42) 


If < < 1, the sine function may be expanded 

4?* 2\r 

to obtain 


Po = 


TTrt^pa^, 

AT 


(43) 


where p^ is used to indicate the pressure at distances 
where 

r>> — and^>>l. (44) 

A 

In this region, the inverse-square law does hold, and 
the pressure does vary inversely with r. 


54 


TESTING TECHNIQUE 



Figure 14. Spherical wave correction for circular piston. Correction to be added to measured response. 


I’he deviation from the inverse-square law may be 
treated as a correction. What one would like to meas- 
ure is pQ as given by equation (43). Thus, the ratio 
of the measured pressure to the desired result is 


and 

d>2L (48) 

where L is the longest dimension of the piston. 


t 

Po 


7ra^ 


sin |(V«“ + ’■- 


r) 


(45) 


Substituting for r the usual symbol for testing dis- 
tance d, and for a, the diameter of the piston D/2, 
this equation becomes 


(46) 

A chart is given in Figure 14 showing the corrections 
in db to be added to the measured values. A similar 
analysis may be made for a line transducer. The cor- 
rections for this instrument are shown in Figure 15 
where L, the length of the line, replaces D. 

While the preceding derivation is for a circular 
piston, essentially the same limit holds for a piston 
of any shape if the diameter of the piston is replaced 
by a characteristic linear dimension for the shape 
under consideration. A general criterion for the 
domain where the inverse-square law is valid for a 
piston is given by 

d>y ( 47 ) 


p _ 8xd 
po ttD- 


sm - 
A 


Wf— ) 


5.4.5 Proximity Effect for Pistons: 

Directivity 

4 he effect of proximity on the directivity pattern 
of a transducer is not as amenable to calculation as 
the effect on the axial response. To avoid appreciable 
effect due to proximity in directivity measurements, 
the following criteria should essentially be met: 



and 

d>\0L. (49) 

Qualitatively, the effects on directivity patterns of 
measuring at a closer distance than prescribed by the 
above criteria are known. Measurements show that 
the measured beam width is broader than that found 
at distances in the inverse-square-law region. The 
side lobes of the pattern appear higher than those for 
long distances, and the minima separating the vari- 
ous lobes begin to fill in. These effects are shown for 
an extreme case in Figure 16, where testing distances 


CHOICE OF TESTING GEOMETRY 


55 



Figure 15. Spherical wave correction for a uniform line. Correction to be added to measured response. 


of 1.5 feet and 9 feet are compared. The critical dis- 
tance for the projector tested in this example is 
about 5 feet. 

AVhen measuring axial response, the first criterion, 
d > L^/\, is the more important practical one, but 
for directivity patterns the second criterion, d > lOL, 
is about equally important. This results from the 
fact that the directivity pattern of a device is due 
largely to cancellation at certain angles of in-phase 
and out-of-phase pressures bn the active surface. Close 
in, the inverse-square-law effect makes the amplitude 
smaller on the more distant parts of the transducer, 
so that cancellation is not so effective. This effect is 
particularly troublesome for a long-line hydrophone, 
since the testing distance must be very great to keep 
the amplitude at the two ends of the line approxi- 
mately equal when the line has a radial orientation 
with respect to the source or receiver. 

Other Proximity Effects 

Another proximity effect which occurs with direc- 
tional transducers is connected with their beam pat- 
terns. If the beam pattern of one is quite sharp, it 
may allow an appreciable variation of amplitude 
over the face of the other, independent of the inverse- 
square-law effect. When a directional source is used, 
the variation in pressure over the area of the oppos- 


ing transducer should certainly be less than 1 db if 
the proper response is to be obtained. 

When two transducers face each other during a 
test, the sound field at the receiver includes the 
doubly diffracted (or reflected) field produced by the 
original wave from the source being diffracted by 
the receiver and then rediffracted by the source back 
to the receiver. This effect is usually negligible, ex- 
cept when two transducers of large area oppose each 
other at a short distance, in which case a severe stand- 
ing wave pattern may be set up between them. The 
standing waves may even be of sufficient magnitude 
to change the apparent acoustic impedance of the 
medium as viewed by the source. While this factor 
is rarely a cause of trouble, it should be kept in mind 
when working with transducers of large area. 

Finally, it should be remembered that in a test the 
proximity effects for both instruments must be con- 
sidered and allowance made for the nature of each 
in selecting the testing distance. 

^ Correction for Proximity Effects 

The fact that the error due to a spherical wave 
front can be calculated for many commonly occurring 
cases, such as the pressure-gradient transducer, the 
circular piston, and the line, suggests that correc- 
tions can be made for this effect. Such is indeed the 


56 


TESTING TECHNIQUE 




Figure 16. Effect of testing distance on directivity pattern. (A) testing distance = 9 ft, (B) testing distance 1.5 ft. 
For this transducer L‘/\ = 5 ft. 


case but, as in all other cases where corrections arc 
applied, caution is necessary. The calculations are 
carried out on the basis of the transducer’s behaving 
in a certain well-defined manner. For example, it is 
assumed that all parts of a piston move with the same 
velocity and in the same phase. Actual instruments 
only approximate this behavior and in many cases 
depart significantly from it. The amplitude of a 
piston is often smaller at the edge than at the center, 
or the piston may actually break up into areas which 
oscillate out of phase. Instead of having uniform 
sensitivity, lines are often made up of an array of 
discrete elements and are often shaded or tapered, 
that is, have intentionally reduced sensitivity at the 
ends in order to suppress side lobes. The validity of 
the theoretical formidas is then cpiestionable. 

One must conclude, as a general rule, that a theo- 
retical correction of more than 5 db is open to con- 
siderable question, even if all other criteria as to 
applicability of the theoretical correction are favor- 
able. If possible, test conditions should be selected so 
that corrections for spherical wave effects are avoided. 


When corrections must be made, the charts in Fig- 
ures 13, 14, and 15 giving the corrections for a pres- 
sure-gradient device, a circular piston, and a line will 
be found useful. 


Summary of Testing Geometry 

The important factors which determine the testing 
geometry to be used in calibration measurements 
have now been discussed. It remains to summarize a 
procedure for selecting the optimum testing distance 
and testing depth. An outline is given in the follow- 
ing. 

1. Select the testing depth. If the reflection coeffi- 
cient of the bottom is known. Figure 12 may be used. 
If the coefficient is not known, a testing depth of from 
1/2 to ^ the water depth is usually satisfactory. If the 
water is very deep, the greatest depth consistent with 
satisfactory rigging of the instruments is desirable. 

2. If both instruments are nondirectional, as small 
a testing distance as possible is desirable. It should 


CHOICE OF TESTING GEOMETRY 


57 


not, however, be smaller than several times the larg- 
est dimension of either instrument. Surface screens 
and bottom screens may be helpful in reducing re- 
flections. If the instruments have a relatively flat 
response, that is, low Q or time constant, pulses may 
be used to advantage with a possible increase of test- 
ing distance. 

3. If one of the instruments is a pressure-gradient 
device and the other nondirectional, and if no spher- 
ical wave correction is to be applied, the testing dis- 
tance should be greater than half a wave length. If 
a spherical wave correction is to be applied to the 
results, it should not be greater than 5 db if it is pos- 
sible to avoid it. This means the testing distance 
should exceed %p. of ^ wave length. As a rule reflec- 
tions are not severe until the testing distance is 
greater than the depth, and surface screens may in- 
crease this distance. This means that reasonably good 
calibrations can be obtained down to frequencies 
where the wave length is about ten times the depth. 

4. If two pressure-gradient instruments are used, 
the testing distance should not be less than a wave 
length, since closer to a transmitter of this type the 
pressure gradient is no longer radial. If it is necessary 
to work closer, a correction can be computed and ap- 
plied to the results. 

5. When a directional transducer of the piston 
type and a nondirectional instrument are used, the 
testing distance selected should be such that errors 
due to proximity effects are about equal to errors due 
to reflection interference. The possible variations in 
this case are many and depend on the size of the 
transducer and the frequency, but by an examina- 
tion of Figures 4 and 14 one usually can reach a 
reasonable compromise. If possible, the reflected in- 
tensity given in Figure 4 should be 10 db or more 
down. To avoid spherical wave corrections, the test- 
ing distance should be larger than D-jX. If it is nec- 
essary to use shorter distances, a spherical wave cor- 
rection from Figure 14 may be applied, but any 
correction greater than 5 db must be considered un- 
reliable. The pulse technique may be of value in such 
tests. 

6. The same considerations given in the preceding 
paragraph apply to a pressure-gradient instrument 
facing a directional transducer of the piston type, ex- 
cept that a different spherical wave correction must 
be applied. The fact that the pressure-gradient in- 
strument has directionality itself reduces reflection 
interference difficulties. The reflected intensity is be- 


low the direct intensity in decibels by the sum of the 
values obtained for each instrument from Figure 4. 

7. When two piston-type transducers face each 
other, reflection interference is not usually a source 
of trouble because of the directivity of the trans- 
ducers, except at very great separations. On the other 
hand, proximity effects become of greater signifi- 
cance, and the separation should be great enough so 
that: 


a. Spherical wave effects are small. This requires 
that any testing distance d satisfy the conditions 


and 


d> 


. A 


+ 


6Z)iZ)2 


d > 2/)i 

d > 2 D 2 , 


(50) 


where Di and D 2 are the diameters of the two 
pistons. This also keeps the pressure due to one 
transducer uniform over the face of the other in 
spite of the directivity of the instruments, 
b. No standing wave pattern is formed between 
the faces of the two instruments. This generally 
is taken care of if the criteria in the preceding 
paragraph are met. 


When measuring directivity patterns, the last two 
conditions of equation (50) should be changed to 
d > lODi and d > IOD 2 to avoid the effect of vary- 
ing pressure due to the inverse-square law. 

8. Essentially the same considerations apply to line 
instruments as apply to piston-type transducers pro- 
vided one substitutes the length of the line for the 
diameter of the piston. If the line is suspended ver- 
tically, there are ordinarily no great difficulties in 
obtaining a calibration except possibly at low fre- 
quencies. Spherical wave corrections are useful when 
a testing distance sufficiently great to eliminate prox- 
imity effects cannot be used. If a line is suspended 
horizontally, surface reflection usually causes great 
difficulty and makes it almost impossible to obtain 
good directivity patterns except with great testing 
depths. The inverse-square-law effect also makes it 
difficult to obtain good directivity patterns except at 
great distances, which are consequent on great depth. 
The pulsing technique may be of aid, but it must be 
remembered that, with the line mounted end-on with 
respect to the source or receiver, the acoustic time 


58 


TESTING TECHNIQUE 


constant is at least the length of the line divided by 
the velocity of sound. Unless deep water is available, 
this time may be greater than the time required for 
an interfering reflected pulse to appear. 

While the outline given above should be of aid in 
fixing the geometry for a test, it must be remembered 
that each test has its own particular factors involved 
and thus no general rule can be made. Experience 
and judgment are of primary value in making a 
proper choice. It is nearly always helpful to repeat 
tests with a changed geometry, particularly when it 
is suspected that reflections or proximity effects are 
causing trouble. One then has an internal check on 
the validity of the corrections that may have been 
applied, as well as a means of recognizing the pres- 
ence of interfering reflections or other extraneous 
effects. 

5 5 the establishment of 

SOUND FIELDS 

^ Absolute and Relative Calibrations 

The calibration of underwater sound devices re- 
quires reliable knowledge of the magnitude of sound 
fields in water. Two techniques for establishing these 
magnitudes may be distinguished. One involves a 
direct absolute measurement of the field intensity; 
the other uses previously calibrated instruments eith- 
er to establish a known sound field or to measure one 
which is present. Among the available methods of 
the first type are the Rayleigh disk method, the radia- 
tion pressure method, the reciprocity method, and 
various motional impedance methods. The second 
technique requires calibrated instruments, whose 
calibrations have been obtained either by some of 
the absolute methods mentioned above, or by such 
methods as computation from their design or cali- 
bration in air. In the routine calibration and testing 
of underwater sound devices, the method of relative 
calibration with a known standard is far more con- 
venient than a direct absolute calibration, which re- 
quires considerable care and is more time-consuming. 
It has the disadvantage that it is based on the stability 
of the reference standard, but numbers of these 
standards have been constructed whose calibration 
remains sufficiently constant for the accuracy usually 
desired in relative calibrations. The most satisfactory 
absolute method in water is the reciprocity method, 
and it is now exclusively used by USRL for the abso- 


lute calibration of standards. In the following sec- 
tions the various methods of obtaining absolute 
calibrations are discussed. 

Absolute Calibration from 
Design of Standard 

In principle, if the design of a transducer is com- 
pletely specihed, one can theoretically calibrate the 
device over its entire frequency range by solving the 
equations of acoustics, mechanics, and electromag- 
netism involved in its operation. Actually, the equa- 
tions are too complicated to allow a practical solution 
unless some assumptions are made. In spite of these 
approximations, it may often be possible to obtain a 
reasonably valid theoretical response characteristic 
for the device over a considerable range of frequen- 
cies. This method has been applied with consider- 
able success to certain standards in use at USRL, in 
particular to the 3A Rochelle salt crystal hydrophone 
and to the lA pressure-gradient type hydrophone 
designed by the Bell Telephone Laboratories.^- The 
principal difficulties in the method are the following: 

1. It is not feasible to take into account all pos- 
sible modes of vibration of the device and its housing, 
but only the desired mode and perhaps a few closely- 
coupled ones. However, some of the neglected modes 
are excited in operation, particularly near their 
resonant frequencies, and may introduce “break-ups” 
in the response which will not be included in the 
computed calibration. 

2. At all frequencies, but particularly at those hav- 
ing wave lengths of the order of magnitude of the 
dimensions of the device and higher, diffraction 
around the instrument plays an important role. This 
diffraction effect is very difficult to compute, and 
computations have been carried out only in some 
highly idealized cases. Thus, it is difficult to include 
in the theoretical calibration precisely the effect of 
diffraction. 

3. Some of the constants of the mechanical ele- 
ments involved in the construction of an instrument 
are not easily measurable. In particular, mechanical 
resistance as a function of frequency as well as of the 
effective mass and stiffness of various elements may 
be difficult to obtain over the desired frequency 
range. 

4. The method can be applied only to the rela- 
tively few instruments designed with this method of 
calibration in view, and then only by skilled person- 


ESTABLISHMENT OF SOUND FIELDS 


59 


nel intimately familiar with every phase of the de- 
sign. 

5. The reliability of calibration obtained in this 
way is always open to question, unless a check can 
be obtained by other methods. 

6. The frequency range which may be covered is 
limited. 

One must conclude, consequently, that this is not 
a very satisfactory method of absolute calibration, 
although its principles are essential to the design of 
satisfactory reference standards. 

5.5.3 Absolute Calibration from 

Calibration in Air 

Initially, the technique of calibration of acoustic 
devices in air was developed considerably beyond 
that in water. If one could obtain the calibration of 
a device in water from its calibration in air, one 
would have a useful method of calibration for under- 
water sound devices. The mechanical and electrical 
elements of a transducer are not functions of the 
medium in which it is immersed. Therefore, it is 
necessary to consider only the effect of a change of 
medium on the acoustic elements. The important 
parameters are the density and the sound velocity. If 
a device is essentially pressure-actuated, the voltage 
which it develops in a sound field is proportional to 
the pressure properly integrated over its surface, in- 
cluding the pressure due to diffraction. At wave 
lengths where diffraction is negligible for a stiff‘d de- 
vice, the pressure acting is simply the pressure in the 
field, so that the acoustic pressure is the same regard- 
less of the medium. Thus, at low frequencies a stiff 
pressure-actuated transducer has the same receiving 
response in air and in water. The upper frequency 
limit for this equality is determined by the frequency 
at which diffraction becomes important. This fre- 
quency is 1/4 to % as high in air as in water because 
of the 4.3 to 1 ratio of the velocities of sound in the 
two media. For a device of the order of 1 inch in size, 
the frequency at which diffraction becomes important 
is about 12 kc in air and about 60 kc in water. Thus, 
for a pressure-operated device of this type, such as the 
3A hydrophone, a water calibration up to about 10 
kc can be obtained from an air calibration, and, by 

(I By “stiff” is meant that, at the frequencies of interest, the 
radiation impedance is small compared to the mechanical im- 
pedance of the transducer. 


a judicious frequency translation of diffraction effect, 
the calibration may often be extended higher. 

For a pressure-gradient operated instrument, the 
response is essentially proportional to the pressure 
gradient in the sound field. Now, for the same fre- 
quency and pressure in a plane wave in air and in 
water, the ratio of the pressure gradient in air to that 
in water is equal to the ratio of the sound velocity 
in water to that in air. If this were the only effect, a 
pressure-gradient transducer would be 20 log 4800/ 
1100 = 12.7 db more sensitive in air than in water at 
the same frequency. However, in such devices the 
change in radiation impedance with change of me- 
dium is often not negligible. The radiation reactance 
of the transducer is a function of the density of the 
medium, and the functional dependence is different 
for different instruments. This effect must also be 
taken into account. For the 1 A hydrophone, the addi- 
tional correction amounts to 3.3 db, making the 
hydrophone 16 db less sensitive in water than in air 
at the same frequency. In addition, diffraction effects 
become a factor in the response at different fre- 
quencies for the two media, as pointed out above for 
pressure-type instruments. 

Thus, a calibration in aiF^-^^ can be used to give a 
calibration in water only over a limited frequency 
range. The original calibrations of USRL standards 
before the reciprocity method was adopted were ob- 
tained in this fashion. 

While, with judicious treatment of the data, these 
methods can give reasonably good calibrations over 
the most important part of the frequency spectrum 
for underwater sound work, they have distinct dis- 
advantages: 

1. They are relatively laborious and require for 
their proper execution intimate knowledge of the 
instrument, as well as personnel highly skilled in the 
technique of air calibration and in the principles of 
acoustic design. 

2. They can be applied only to relatively few in- 
struments which are designed with this method of 
calibration in view. 

3. There is no check on their reliability and no 
positive assurance that the various modes of vibra- 
tion of the device may not be excited to different de- 
grees in air and in water. 

4. Theoretical corrections, whose validity may be 
questionable, must be applied to the results. 

5. The frequency range which may be covered is 
limited. 


60 


TESTING TECHNIQUE 


5.5.4 Quasi-Static Calibration 

For an acoustically stiff pressure-operated instru- 
ment at frequencies low enough so that the wave 
length is long compared to the dimensions of the 
instrument, the response depends only on the pres- 
sure in the neighborhood of the instrument. It is 
independent of the type of wave giving rise to the 
pressure, of its direction of propagation, and even 
of whether there is a wave present at all, so long as a 
hydrostatic pressure variation of corresponding am- 
plitude and frequency is present. To calibrate such 
an instrument in this low-frequency register, it is 
necessary only to produce a known pressure variation 
in the portion of the medium adjoining the instru- 
ment. Several possible methods of calibration are 
based on this principle. 

For frequencies below a few cycles per second, one 
can bring about this pressure variation in water 
simply by raising and lowering the device sinusoid- 
ally through a known distance at the desired fre- 
quency. If the depth in centimeters is h, then the rms 
pressure in dynes per sq cm acting on the transducer 
is 




Pgh 

2V2 


( 51 ) 


where p is the density of water (1 gram per cu cm) and 
g is the acceleration of gravity (980 cm per sec per 
sec). If the test is carefully made, accurate calibra- 
tions in this low-frequency range can be made. 

At higher frequencies, it is possible to use a tank 
in which the pressure is varied sinusoidally through 
known values. This can be effected by building what 
is essentially a low-frequency transmitter into the 
wall of a closed stiff tank. If the transmitter is of the 
electromagnetic type, whose stiffness is low compared 
to the stiffness of the tank in the frequency range of 
interest, then the force exerted by the piston can be 
calculated from the current into the projector, either 
by measuring or by calculating the force per unit 
current developed by the piston when it is blocked. 
When this force is known, the pressure which it pro- 
duces in an acoustically stiff chamber can be calcu- 
lated. 

This method is applicable only for frequencies 
far below those for the first chamber resonance of the 
tank, since, as the tank approaches its lowest reson- 
ance, its stiffness drops in value, becoming quite low 


at the first resonance. Also, at this point, pressure 
can no longer be considered uniform throughout the 
tank, and consequently one cannot calculate in any 
simple manner the pressure at any desired point from 
the force exerted by the piston. If the walls of the 
chamber have resonances below the cavity resonance 
of the chamber, these resonant frequencies reduce 
even further the upper limit of the useful frequency 
range. This method has been used with considerable 
success by USRL with a tank of the type described, 
built by the Bell Telephone Laboratories.^*-^ The low- 
est resonance frequency for this system is about 300 
c, so that the system is useful up to about 100 c. In 
using such a system, one must remember to avoid 
any condition which lowers the stiffness of the 
chamber, such as the presence of air bubbles or an 
acoustically “soft” transducer. Over a limited range, 
corrections may be made for decreased stiffness due 
to any effect, provided this stiffness can be measured. 

Other quasi-static absolute calibration methods 
have been employed. One makes use of a condenser- 
type hydrophone^® in which the capacitance of the 
condenser is in one arm of an impedance bridge em- 
ploying a carrier frequency of 5 kc. Changes in pres- 
sure on the diaphragm cause a variation in capaci- 
tance which unbalances the bridge. The amount of 
unbalance becomes a measure of the pressure. This 
particular system is flat from 0 to 75 c, above which 
the effect of the first resonance of the hydrophone 
becomes prominent. Hydrostatic pressure equaliza- 
tion is provided to eliminate the variation of calibra- 
tion with hydrostatic pressure. If, however, the 
equalization cannot be carried out, then, by lowering 
the hydrophone a known distance in water, the abso- 
lute calibration can be obtained from the bridge 
unbalance thus produced. Since the hydrophone is 
known to have a flat response up to 75 c, this direct 
hydrostatic pressure calibration is applicable over 
this range. 

5.5.5 Absolute Methods Not Involving 
Transducers 

There are several methods of establishing abso- 
lutely the magnitude of a sound field without the use 
of an electroacoustic transducer. Among these may 
be listed the Rayleigh disk method, the radiation 
pressure method, and optical methods. These all re- 
quire relatively delicate measurements which, while 
difficult to perform in air, are even more difficult in 


ESTABLISHMENT OF SOUND FIELDS 


61 


water. They are, therefore, of negligible practical 
significance in underwater sound calibrations. 

As evidence of this, consider the use of a Rayleigh 
disk. This is a thin circular disk suspended from a 
fine torsion filament so that the plane of the disk 
makes a definite angle 6 with the direction of propa- 
gation of the sound wave. If the wave length is much 
greater than the diameter of the disk, there is a torque 
exerted on the disk of magnitude 

Af = I sin 2 e (52) 

6 

where p is the density of the medium, a the radius of 
the disk, and v the rms particle velocity. Expressed 
in terms of the pressure, this becomes 

M = I sin 2 0, (53) 

5 pC“ 

c being the velocity of sound. Thus, for the same pres- 
sure in air and water, the ratio of the torque in water 
to that in air will be 


^ _ PaCq^ 
o Pw 


6.7 X 10-5. 


(54) 


Since torques obtained with the Rayleigh disk are 
very difficult to measure in air for any reasonable 
sound pressures, one sees that it would be almost im- 
possible to measure them in water, even if the experi- 
mental difficulties in setting up the apparatus could 
be overcome. The Rayleigh disk and similar methods 
must be discarded, therefore, as impractical for abso- 
lute calibration in water. 

Radiation pressure is essentially the steady pres- 
sure exerted on a surface when sound is reflected 
from the surface and is, like the torque in the Ray- 
leigh disk, a second order effect. If a plane sound 
wave strikes normally a completely reflecting surface, 
the area of which is numerically much greater than 
the wave length, the radiation pressure on the surface 
is given by 

P = (55) 


where k is the ratio of specific heats for the medium 
(practically unity for liquids), p is the rms sound 
pressure, p the density of the medium, and c the veloc- 
ity of sound. For a sound pressure of 1 dyne per sq cm 
in water, the radiation pressure would be about 10“ 


dyne per sq cm. Obviously, for such low-pressure 
measurements, very delicate apparatus is required, 
so that the method is usually of little practical value 
although it can be used in the laboratory with high 
sound pressures such as may be developed by quartz 
crystals at high frequencies. 

There are various other methods of calibration 
characterized by the fact that an electroacoustic trans- 
ducer is not used, but all are more or less subject to 
the objection that the measurements are exceedingly 
delicate. Some are based on the variation of the index 
of refraction of a fluid with pressure or similar ef- 
fects. They usually have a limited frequency range 
over which they can be employed. In comparison 
with the methods of calibration which can be per- 
formed with relative ease, none of them has much 
practical importance at the present time. Further in- 
formation regarding them may be obtained by con- 
sulting various reference works on acoustics and the 
general acoustical literature. 

5.5.6 Yhe Reciprocity Method of 
Calibration 

By far the most accurate, simple, and generally 
useful method of absolute calibration is the so-called 
reciprocity method, based on the reciprocity princi- 
ple as applied to electroacoustic transducers. Once 
its advantages are enumerated, the reasons for its 
adoption as a standard method of obtaining absolute 
calibrations by USRL are clear. These advantages 
are: 

1. The method is apparently applicable over the 
entire practical range of frequencies. 

2. The actual measurements are easily made and 
are essentially similar to those employed in relative 
calibrations (comparison method). 

3. The method can be used by relatively unskilled 
personnel. 

4. The measurements may be carried out in the 
field rapidly and easily. 

5. Though the accuracy of the method is at present 
limited by reflection-interference difficulties, these 
also limit the accuracy of comparison methods. 

6. If certain easily attainable conditions are main- 
tained, there are no theoretical corrections to be ap- 
plied to the results. 

7. Several independent calibrations can be per- 
formed at the same time, giving an immediate check 
on the accuracy of the results. 


62 


TESTING TECHNIQUE 


The reciprocity method is based on the fact that for 
a passive linear electroacoustic transducer which 
obeys the reciprocity principle (this includes most 
transducers now in use), a definite and simple rela- 
tionship holds between its response as a transmitter 
and as a receiver. Consider an electroacoustic trans- 
ducer with its acoustic center (which may be arbitrar- 
ily selected) located at a point Fo- The transducer is 
being operated as a transmitter with a current 1 flow- 
ing into it. At some other arbitrary point P the trans- 
ducer produces a pressure p. Let S be the transmitting 
sensitivity expressed as the ratio of the pressure at P 
to the current / (that is, S = pjl). Suppose that there 
is placed at the point P the center of a source of spheri- 
cal waves, and let pc be the pressure produced at the 
point Pq when the transducer is not present. If the 
transducer is present, it develops in this sound field 
an open-circuit voltage E across its terminals. Let 
the ratio of E to pc be the receiving sensitivity M 
(M = Ejpc). The reciprocity principle then states that 

^ = 2*^ (56) 

S pc ^ ^ 

where d is the distance from P to P^, X is the wave 
length, and p and c are respectively the density and 
sound velocity of the medium in which the trans- 
ducer is present. 

To make use of the reciprocity principle for the ab- 
solute calibration of a transducer, three transducers 
are employed. One is used only as a transmitter or 
projector, a second only as a receiver, while the third 
must be a reversible transducer obeying the reciproc- 
ity principle. The projector is placed at a point P 
with a definite orientation. The receiver is placed at 
a point P' sufficiently distant from P so that the waves 
reaching it from the projector are essentially plane. 
I'hen for a given current Ip in the projector, the volt- 
age developed by the receiver is measured, either on 
open-circuit or across an impedance kept constant 
during the measurements. This voltage is denoted by 
Epp. The receiver is now replaced by the transducer 
and the open-circuit voltage developed by it, E^p, is 
measured for the same current Ip in the projector. 
Next, the projector is replaced by the receiver, whose 
orientation with respect to the transducer must be the 
same as it was previously with respect to the projector. 
The transducer is then operated as a transmitter with 
a current Ip, and the voltage developed by the re- 
ceiver E.pp is obtained. 


If Mp denotes the sensitivity of the receiver and Mp 
the open-circuit receiving sensitivity of the trans- 
ducer, then, since the pressure developed by the pro- 
jector is the same in the first two trials. 


_ Epp 

p Epp 


(57) 


If Sp is the transmitting sensitivity of the trans- 
ducer, that is, the ratio of the pressure produced at 
the receiver to the current Ip in the transducer, then 

Epp = MpSpIp (^^) 

since Spip is the pressure at the receiver. From the 
reciprocity principle 


Mp ^ 2d\ 
Sp pc 


(59) 


where d is the distance from P to P'. Substituting the 
value of Sp from equation (59) in (58), 




(60) 


Eliminating Mp between equations (60) and (57), 


p _ ]\/[ i^R^Tp) J (pd) 

r,pT — iVLp — Ip — 


P, 


2dX 


(61) 


M. 


^ ^ ^ ^ 2dxY 

\_Epp Ip pc J 


(62) 


In this way, the calibration of the hydrophone H is 
expressed in terms of measured quantities. The pre- 
ceding discussion assumes cgs units for all quantities 
thus including absolute electrical units. If volts and 
amperes are used in equation (62), Epp must be mul- 
tiplied by lO’^. Thus, the absolute calibration may 
be obtained by a process which involves only elec- 
trical measurements on the transducers. 


5.5.7 Notes on the Reciprocity Method 

To obviate any misunderstanding concerning the 
use of the reciprocity method of calibration, the fol- 
lowing notes are added: 

1 . The open-circuit voltage developed by the trans- 
ducer T operating as a receiver should be measured 


ESTABLISHMENT OF SOUND FIELDS 


63 


at the same terminals at which the current into the 
transducer is measured when operating as a trans- 
mitter. The choice of the terminals can be arbitrary 
to a considerable extent. They may be directly at the 
electric output of the transducer element itself (for 
example, the crystal, in a transducer of that type) or 
at the end of a considerable length of cable. In fact, 
two terminals of a four-terminal passive electric net- 
work may be connected to the transducer, and the 
terminals used in the calibration selected as the re- 
maining pair of terminals of the network. Any of 
these conditions is satisfactory, provided the same 
pair of terminals is used for both current and open- 
circuit voltage measurement. 

2. It is specified above that the distance d should 
be large enough so that waves from either the pro- 
jector P or transducer T (when transmitting) are 
effectively plane at this distance. This is necessary 
if the plane wave calibration of the hydrophone H 
is desired. The distance may be shortened if one 
wishes to obtain a calibration for H in terms of 
spherical waves of a given radius, or if one can apply 
a spherical wave correction to reduce a spherical wave 
calibration to a plane wave calibration. The latter 
procedure may be necessary if reflection interference 
is present to a degree which seriously interferes with 
the accuracy of the measurements. The presence of 
reflections introduces the same inaccuracies in a re- 
ciprocity calibration as in comparison tests, and the 
methods described in Section 5.3 for eliminating re- 
flection interference may often be profitably applied 
in reciprocity calibration tests. 

3. We have indicated that the choice of the acous- 
tic center of the transducers was arbitrary in the pre- 
ceding discussion, yet the distance d between centers 
enters explicitly into the formula for the calibration. 
This can be understood if one remembers that the 
receiving response also depends upon the choice of 
the center, and this latter dependence on d cancels 
the explicit dependence on d in the formula. See 
Chapter 4, equation (10). Usually the acoustic center 
is chosen close to the geometric center of the instru- 
ment, but in principle one may take it to be any- 
where. If it should be chosen far from the actual 
instrument, the center must be considered as part of 
the instrument in requiring that the wave be essenti- 
ally plane; that is, the plane wave response with such 
a choice for the acoustic center can be obtained only 
if the wave from the transmitters is essentially plane, 
not only over the transducer but over the entire re- 


gion between the center and the transducer. Thus, 
there is an advantage, insofar as choice of testing dis- 
tance is concerned, in selecting the center within, or 
in the immediate neighborhood of, the transducer. 

4. It should be clear that the reciprocity calibra- 
tion of a transducer can be carried through for any 
orientation (direction of sound incidence) of the 
transducers involved, but the same relative orienta- 
tions must be maintained during the series of tests. 
One also sees immediately that the directivity pattern 
of a transducer obeying reciprocity is the same on 
transmitting and on receiving at the same frequency. 

5. One should note that the responses given by the 
formula represent ratios of magnitudes of quantities 
without consideration of phase. To obtain the phase 
of the response, one must include a phase factor, 
which is not ordinarily known but can be deter- 
mined in the reciprocity relation shown in equation 
(56) for the particular reversible transducer. The 
phase of the response is not usually of interest, but in 
some cases it may be important. 

6. In making a reciprocity calibration one must 
have a transducer obeying the reciprocity principle, 
and therefore should have a means of establishing 
this property. While it is known that it is possible to 
have a linear passive reversible transducer which does 
not obey reciprocity, almost all transducers of interest 
do have this property. No generally applicable condi- 
tions have been established to guarantee reciprocity 
in a transducer, but there are some general principles 
which serve as useful guides. Theory seems to indi- 
cate that if the electromechanical coupling is of the 
electromagnetic or magnetostrictive type, or a com- 
bination of these, reciprocity is obeyed. Similarly, 
there are indications that electrostatic or piezoelectric 
coupling or a combination of these also insures reci- 
procity. A parallel combination of one of the first 
group (electromagnetic or magnetostrictive) with one 
of the second (electrostatic of piezoelectric) in general 
leads to a transducer which does not obey reciprocity. 
Since such combinations are rare, most actual trans- 
ducers will apparently obey reciprocity. The condi- 
tion for reciprocity is sufficiently established if the 
efficiency of the transducer is 100 per cent. Since no 
actual transducers attain this efficiency, this criterion 
is of questionable value for practical application. 

One must therefore resort to the criterion of in- 
ternal consistency between the calibrations obtained 
with several reversible transducers as a check that 
they obey the reciprocity principle. It is very unlikely 


64 


TESTING TECHNIQUE 


that, if these reversible transducers did not obey the 
reciprocity principle, one would obtain the same cali- 
bration by the use of each. 

7. If a calibrated resistor is available so that cur- 
rent can be measured by measuring the voltage drop 
across the resistor in series with the transducer, one 
need not have an absolutely calibrated voltmeter to 
perform a reciprocity calibration. For, if one sub- 
stitutes EjjR for It in equation (62), it can be seen 
that only the ratio of voltages, and not their absolute 
magnitudes, enters into the formula. 

5.5.8 Motional Impedance Methods 

There have been proposed and applied several 
methods of absolute calibration, based on the meas- 
urement of the impedance of the transducer, which 
can be applied to transducers obeying the reciprocity 
principle. It is sufficient here to indicate how one of 
these methods leads to an absolute calibration. If 
one considers a transducer which has a sharp me- 
chanical resonance at some frequency, the impedance 
shows a rapid variation with frequency in the neigh- 
borhood of the resonant value. If one plots the re- 
sistance and reactance as a function of frequency, 
smooth curves can be drawn connecting the portions 
of the resistance curve and the reactance curve far 
above and far below resonance. These curves are re- 
ferred to as the blocked resistance and reactance, 
since they correspond to the impedance which would 
be measured if the diaphragm of the transducer were 
prevented from moving. 

The difference between the actual impedance and 
the blocked impedance is referred to as the motional 
impedance. If the motional resistance is plotted 
against the motional reactance in rectangular co- 
ordinates, with frequency as parameter, a figure is 
obtained known as the motional impedance circle. 
If one measures the diameter of the motional im- 
pedance circle in ohms for the instrument immersed 
in water and then in air, calling the quantities D^, 
and respectively, one can show that the efficiency 
E of the transducer in water, that is, the ratio of 
acoustic power output to electric power input, at 
resonance, is given by 

^ ( 63 ) 

where R is the actual resistance of the device in water. 


The directivity pattern and thus the directivity in- 
dex of the transducer can be measured with an un- 
calibrated transducer. From equation (8), Chapter 4, 
one can see that, if the efficiency and directivity index 
of a transducer are known, its response can be deter- 
mined. In this way, the device can be absolutely cali- 
brated at its resonant frequency from the motional 
impedance circle (which gives the efficiency) and a 
directivity pattern (which gives the directivity index). 

Since this method furnishes an absolute calibration 
at only one frequency, it is not of great value as a 
general calibration method, but it is useful for ob- 
taining the efficiency of a device at resonance by 
purely electrical impedance methods. Some of the 
other methods of motional impedance analysis are 
more refined and allow calibration over an extended 
frequency range, but in general they are not so con- 
venient to use as the reciprocity method. 

5.5.9 Relative Calibration of Transducer 

Even the reciprocity method entails more effort 
than is desirable for the calibration of most devices. 
The comparison method, involving the calibration 
of one transducer against another which has already 
been calibrated, provides a practical means for the 
rapid calibration of most devices. In the comparison 
method the magnitude of the sound field is first estab- 
lished by means of a previously calibrated standard. 
This is then followed by the calibration of the device 
to be tested in this known sound held. 

It is presumed in a calibration by comparison that 
the reference standard is sufficiently stable in con- 
struction and operation so that its calibration is re- 
tained in the interim between its own calibration and 
its application in a relative calibration. Either a 
transmitter or a receiver can be used as a reference 
standard, the former to establish a known sound 
held, the latter to measure the magnitude of the 
sound held produced by an uncalibrated transmitter. 
It has been found by USRL that properly constructed 
receivers are somewhat more reliable than transmit- 
ters as reference standards, but the difference is not 
great. In fact, by using both a calibrated receiver and 
a calibrated transmitter in a comparison test, a cross 
check on the stability of the standards may be ob- 
tained in conjunction with the test. 

The procedure in the relative calibration of a re- 
ceiver is the following: A transmitter is placed at one 
point in the water and driven by a constant voltage 


ESTABLISHMENT OF SOUND FIELDS 


65 


(or current) at some frequency. A standard receiver 
is then placed at an appropriate position in the sound 
field, and from its generated voltage the magnitude 
of the sound field can be obtained. The reference 
standard is then replaced by the receiver under test 
and its generated voltage obtained, giving its re- 
sponse at that frequency. The frequency may be 
swept during each test, and then, by comparing meas- 
ured values of the generated voltage of the standard 
and the transducer under test at equal frequencies, 
the frequency response characteristic of the trans- 
ducer may be obtained. 

It should be noted that, if a plane wave calibration 
of the instrument is desired, then, at the position se- 
lected for the receivers, the waves from the transmit- 
ter must be essentially plane with respect to the 
instruments. \Vhether or not this is the case depends, 
among other things, on the nature of the instruments 
themselves, particularly on their size. Because of these 
conditions, the waves at one point may be essentially 
plane for one receiver but not for the other. In this 
case, it may be more convenient to test the two instru- 
ments at different distances from the source. This 
may be done, provided both distances lie in the in- 
verse-square-law region for the source, so that only an 
inverse-square-law correction need be applied. In 
some cases one may find it desirable or expedient, 
because of the presence of reflection interference, to 
operate the instruments at closer distances and to 
apply a spherical wave correction to the result, as has 
been described previously. 

In calibrating a transmitter, one uses a calibrated 
standard receiver which is located in the inverse- 
square-law region of the field of the transmitter. The 
pressure produced at this point can then be deter- 
mined from the voltage generated by the receiver. 
Again, it may be desirable or expedient to locate the 
receiver closer to the transmitter and apply a spheri- 
cal wave correction. 

Since a relative calibration is based on the stability 
of calibration of the standard, frequent checks on the 
calibration of the standards must form a regular part 
of any testing program extending over a long period 
of time. These checks are most conveniently made at 
regular intervals by means of the reciprocity method. 
The reciprocity method is particularly valuable for 
this purpose at a test station since the procedures used 
are the same as for relative calibrations. Thus the 
equipment necessary for a reciprocity calibration is 
available at all times and no extra equipment is neces- 


sary. A running check on all tests may be made by 
using several standards and checking the calibrations 
thus obtained against each other. A calibrated trans- 
mitter may serve as one of the standards. 

5.5.10 Choice of Standards 

For maximum accuracy, reliability, and general 
versatility, a standard should possess certain char- 
acteristics which are outlined below. 

Stability With Time 

Stability is essential, since the accuracy of a relative 
calibration is limited by any change in the response 
of the standard from the time that it was calibrated 
to the time of its use. For this reason, it should be suf- 
ficiently rugged so that slight jars do not change its 
calibration, and it should be constructed of materials 
whose properties do not change with time. If it con- 
tains permanent magnets, their flux density should 
be permanent. The stiffness of springs should not 
vary as a result of aging, or of cold working resulting 
from their extension and retraction. The instrument 
should be constructed so that dampness or moderate 
heat or cold do not change its calibration, and all ex- 
posed parts should be resistant to corrosion. 

Temperature Independence 

It is desirable that the response of a standard be 
independent of temperature over the useful fre- 
quency range of the instrument, since the tempera- 
ture rarely can be controlled in calibration tests. 
Otherwise, it is necessary to know how the calibration 
varies with temperature, which would require con- 
siderable additional labor. Temperature dependence 
of response is an important factor in working with 
Rochelle ^alt crystal devices because of the rapid 
variation of the dielectric constant of x-cut crystals 
and consequent change of impedance in the neigh- 
borhood of the upper Curie point (24 C or 75 F). This 
temperature variation does not have a serious effect 
upon the response of a Rochelle salt crystal receiver 
if the impedance into which the crystal operates is 
high compared to its own impedance. For a Rochelle 
salt crystal transmitter, the response expressed in 
terms of pressure per unit current input does not vary 
appreciably with temperature, although the response 
expressed on a per volt or per available watt basis 
may vary greatly. It is therefore desirable to operate 
Rochelle salt crystal projectors on a constant current 


66 


TESTING TECHNIQUE 


basis, which may be accomplished by making the 
source of electrical power have a high output im- 
pedance compared to the impedance of the crystal. 

W'ide-Banu Uniform Response 

It is desirable that a standard have a smooth fre- 
quency response over a wide frequency band. Rapid 
variations in response with frequency make it diffi- 
cult to compare the responses of two instruments in 
such a range because of the difficulty of reading ac- 
curately a steep curve on a recorder chart. A wide 
frequency band is desirable so that the number of 
standards required to cover the entire frequency 
range of interest be as small as possible to reduce 
rigging time. For high-frequency transmitters it is 
difficult to obtain a flat frequency response. However, 
a smoothly varying response is in general satisfactory 
and can be obtained readily with crystal projectors. 

Linearity, Large Dynamic Range 

Over the range of pressures which a standard is re- 
quired to measure or to produce, the device should 
be linear; that is, the voltage produced by a receiver 
at each frequency should be proportional to the pres- 
sure of the sound field in which it is contained, and 
the pressure produced by a transmitter at some point 
in the field at each frequency should be proportional 
to the input current or voltage applied to it. AVhen 
nonlinearity occurs, a sinusoidal input signal (elec- 
tric or acoustic) no longer, in general, produces a 
sinusoidal output of the same frequency. Most instru- 
ments, however, are linear o\er a limited range of 
input signal amplitudes. For the greatest usefulness 
of a standard, this dynamic range should be as great 
as possible, as this allows a large range of sound field 
pressures to be produced or measured by a single in- 
strument. Since a transmitter may have variations in 
response with frequency of 50 db or more, in order 
to make a comparison calibration with a single stand- 
ard receiver, the standard must have a corresponding 
dynamic range as limited by nonlinearity on the high 
end and inherent noise on the low end. 

Low I'hreshoed 

Hie lowest pressure that can be measured by a re- 
ceiver is determined by its inherent noise voltage. 
I'his may be due to thermal noise, vacuum-tube 
noise (of an associated preamplifier), contact noise, 
or other similar factors. At each frequency, the pres- 
sure at which the signal voltage of the instrument is 


equal to the noise voltage, in a 1-c band centered at 
the frequency, is known as the threshold of the trans- 
ducer. It is desirable to have the threshold of a stand- 
ard as low as possible in order to extend the dynamic 
range as far as possible in the direction of low pres- 
sure. 

Reasonably High Response 

The magnitude of the response of a standard can 
be of considerable importance independent of its 
inherent noise characteristic. For example, if the re- 
sponse is low, the electric crosstalk between a receiver 
and a transmitter may exceed the level of the elec- 
trical signal to be measured. A similar situation ap- 
plies with respect to the use of a transmitter as a 
sound source. Because of the presence of ambient 
noise in the water, it is necessary that the response of 
a transmitter standard be sufficiently great so that its 
sound field exceeds the ambient noise sound field. 

A \ ariety of standard transducers have been devel- 
oped which satisfy the conditions outlined above. 
1 heir characteristics are given in another part of 
this Aolume. 

5 6 CALIBRATION OF DEVICES 

COVERING WIDE FREQUENCY RANGES 

\Vhen a transducer is being used for wide-band 
reception, a single frequency calibration is still sig- 
nificant, because, if the de\ ice is linear, its wide-band 
response can be determined from the single fre- 
quency response by superposition in accordance with 
Fourier’s theory. The calibration then consists in de- 
termining over what range the hydrophone is linear 
and in taking a single frequency characteristic within 
that range. Hie procedure for taking a single fre- 
quency characteristic has been described. The linear- 
ity at any given frequency is best observed by varying 
the input level at that frequency and seeing whether 
or not the output level follow^s proportionately. In- 
stead of taking a single frecpiency characteristic, it 
is, of course, also possible to measure the response for 
a signal with any type of frecpiency spectrum. 

In jiarticular, the response may be measured for a 
signal consisting of very complex aperiodic w^ave 
forms. Such signals usually are referred to as noises. 
The sound created by thermal agitation, the so-called 
thermal noise, is an illustration in point. 

The measurement of such signals places cpiite 
severe recpiirements on the measuring system. It is 


CALIBRATION FOR WIDE FREQUENCY RANGES 


usual to measure the overall signal level and to ob- 
tain a frequency analysis of the noise. Often the time 
variation of the noise is of interest. This can be ob- 
tained in the form of a time-level distribution or as 
the crest factor of the noise. This latter is defined as 
a ratio of the crest value to the effective value of the 
quantity. 

The overall signal level is determined with a wide- 
band measuring circuit. This circuit must carry the 
highest noise peak without overloading and include 
a square-law measuring device, which is the only type 
that adds up the contributions of the various fre- 
quencies in the signal in such a way as to give the rms 
signal level. 

A frequency analysis can be obtained by sweeping 
over the frequency range of the noise with a narrow 
heterodyne band-pass filter. The design of this filter 
must be carefully considered from the standpoint of 
transient response. The requirements for the rest of 
the system are the same as those discussed above for 
measuring the overall signal. 

The crest factor can be measured by obtaining a 
wide-band response with a square-law rectifier (ther- 
mocouple) and by obtaining the peak response on an 
oscilloscope. 

One of the difficulties in all of these measurements 
is the requirement of using a square-law measuring 
device. The only true square-law device is the ther- 
mocouple, but this is so slow-acting that it can furnish 
only a long time average and has a limited dynamic 
range. Therefore, vacuum-tube detectors are usually 
used. These are fast, but they follow the square law 
only over a limited range of input levels. The ques- 


H7 


tion then arises as to whether or not the noise can be 
satisfactorily measured with the particular rectifier 
available. One method that has been used to answer 
this question consists of measuring the crest factor 
and determining whether or not the rectifier is still 
square law for the highest peaks in the noise. While 
this method is helpful, there is theoretically some 
question as to whether it is a sufficient criterion. It is 
therefore desirable to consider the problem from 
other angles as well. Sometimes previous measure- 
ments on similar noises which have been satisfactory 
are available. At other times an assurance can be de- 
veloped from a study of the data itself. 

The above requirements on the measuring system, 
namely, that it (1) shall not overload on the highest 
noise peaks, (2) shall adequately cover the frequency 
range of the noise, and (3) shall have a square-law 
rectifier, are sufficient to obtain a level measurement 
and a frequency analysis of the noise. In some cases 
it is desirable also to obtain a graph of the noise or to 
view it on the oscilloscope. In such cases, it is neces- 
sary that the phase relations between different fre- 
quencies be maintained as well as the magnitudes. 
This adds another requirement, namely, that the sys- 
tem have its phase shift linear with frequency. 

The requirements of the pick-up device used in 
the tests are similar to those on the circuits of the 
system. If the hydrophone has a uniform response 
over the frequency range of interest, which usually 
implies a phase shift linear with frequency, it will be 
satisfactory for use in these tests both for obtaining a 
level measurement and also from the standpoint of 
maintaining phase relations. 


Chapter 6 

DESCRIPTION AND OPERATIONAL PROCEDURES 
OF THE USRL TEST STATIONS 

By Erhard Hartmann and Earle C. Gregg, Jr. 


6 1 DESCRIPTION OF MOUNTAIN LAKES 
TEST STATION 


6 . 1.1 


Site of Station 


T he mountain lakes test station is located on Crys- 
tal Lake in the township of Mountain Lakes, New 
Jersey. This lake is about 650 yards long and 230 
yards wide, and has a small island approximately at 
the center. 

The depth of the lake is fairly uniform, averaging 
about 15 feet. The bottom is a thick stratum of mud. 


which has good sound-absorbing properties, espe- 
cially at supersonic frequencies. The mud, however, 
contains some decomposing organic material which 
produces gas bubbles. Since these are good reflectors 
of sound, the top layer of the mud which contains the 
decomposing material has been removed from the 
bottom of the test areas by dredging. The depth of the 
water in these areas has been accordingly increased 
to about 18 feet. In addition, the lake has been 
treated frequently with copper sulphate to retard 
decomposition and the growth of algae. 


TEST AREA 


TERM BOXES 
REC BOOTH 




4, ^ F SYSTEM 


■■ r 

j istorageI 

> 

it rf' 

* 

‘ L 

1 1 SHED 1 


o — 

— o 


-POWER ROOM 
-PAINT STORAGE 


FiraiRi, 1. Plan view of Mountain l.akes lal)oratoiy anti gronntls. 


68 


DESCRIPTION OF MOUNTAIN LAKES TEST STATION 


69 



Figi're 2. Mew of Mountain Lakes laboratory from Pier 2. 


Facilities 

I'hc Mountain Lakes test station provides facilities 
for the calibration of underwater acoustic devices 
from a fretjuency of 2 cycles })er second to 3.5 mega- 
cycles per second.^' I'his range is covered by means of 
four separate testing systems. A low-frecpiency tank 


a I ransdneers a\ailal)le at present will operate np to 2.2 me. 


system covers the frequency range from 2 e to about 
100 c. Associated with this system are facilities for 
varying the temperature and hydrostatic pressure. 

1 wo intermediate-frequency units, designated sys- 
tem 1 and system 2, are used with outdoor piers for 
free field calibration in the lake. Both systems will 
operate from 15 c to 150 kc and can be arranged to 
test apparatus with power inputs iq^ to 1,500 watts. 
The apparatus has been assembled in bays and ar- 
ranged to provide maximum separation between 
high- and low-level signal paths to minimize cross 
talk. This practice of separation has been maintained 
throughout both systems, including the transmission 
lines and transducer coupling booths on the piers. 
Finally, a high-frequency unit which includes an 
elliptically shaped tank covers the range from 100 kc 
to 2.2 me. This unit may also be used in conjunction 
with one of the piers. 

Other acoustic testing facilities include an open 
tank with sound-absorbing walls (a type used for pro- 
duction testing by the Western Electric Company) 
and a closed cylindrical tank about 15 feet long and 
8 feet in diameter. The latter can be used at hydro- 
static pressures up to 300 lb per s([ in. and at fre- 
(juencies up to about 150 kc. Tests at high pressures 



Figure 3. I esting piers at the Mountain Lakes station. The booths housing terminal equipment and the overhead 
monorail systems are visible. 




70 


USRL TEST STATIONS 



Figure 4. General view of test area of Pier 1. Several suspension carriages and the rotator are visible. Apparatus in 
foreground is being used to make impedance measurements. 


are of interest in connection with underwater sound 
gear used on submarines. 

One of the two outdoor piers is eejuipped for 
handling devices, weighing as much as 2,000 pounds, 
by chain hoists travelling on an overhead rail be- 
tween the loading platform and the far end of the 
pier. The second pier is equipped for devices of not 
more than 250 pounds. For testing distances greater 
than those provided by the piers, a raft with a work- 
ing load capacity of 3,000 pounds is available. Signal 
transmission and power cables extending from the 
laboratory to a position 250 yards out in the lake pro- 
vide facilities for operating equipment on this raft. 

The laboratory has its own machine shop, water 
supply, and heating system. Compressed air and elec- 
tric ]K)wer are available throughout the building. 
Auxiliary equipment includes meters for measuring 
current, voltage, or power; impedance bridges; vac- 


uum-tube test sets; cathode-ray oscilloscopes for 
observing wave shapes; and filters for limiting fre- 
quency bands. 

The electric energy is supplied at 230 volts, 60 
cycles, with the midpoint grounded. This voltage is 
used on the larger motors. The 115-volt supply is 
regulated to keep a uniform voltage on the signal 
generators, amplifiers, detectors, and recorders. 

All high-voltage requirements are supplied by 
regulated rectifiers associated with the various system 
components. Thus, drifts in the calibrating apparatus 
resulting from variation in the supply voltage are 
held at a minimum. A 24-volt d-c supply for the opera- 
tion of relays and indicator lamps is obtained from 
rectifiers energized by the 1 15-volt line. 

Most of the electrical equijjment was designed and 
constructed by the Bell lelej^hone I.aboratories 
under contract with the National Defense Research 



EQUIPMENT AT MOUNTAIN LAKES 


71 


Committee [NDRC]. I’he Underwater Sound Refer- 
ence Laboratories have designed and constructed 
most of the mechanical equipment and have devel- 
oped the high-power equipment, the pulsing system, 
the polar recording system, and certain additional 
features needed for special testing. 

62 CALIBRATION AND TESTING 
EQUIPMENT AT MOUNTAIN LAKES 

Idle calibration and testing systems of USRL are 
described on the basis of their present status. It should 
be emphasized, however, that developments in sonar 
gear and the constantly improving techniques in test- 
ing are making new demands, in many cases beyond 
the capacities of existing equipment. It must there- 
fore be constantly changed and improved to maintain 
the standards required of a reference laboratory. 

For example, system 1, installed in June 1942, in- 
corporated a narrow band-pass filter which discrimin- 
ated against noise and other interference. This was at 
the time a distinct improvement but after the devel- 
opment of the pulse method, which requires a wider 
transmission band, system 1 was inadequate. For this 
reason, and also because of the increasing importance 
of noise analysis, system 1 is now limited in its appli- 
cations, and a continuously increasing proportion of 
the work is handled by system 2. 

* Electrical Components of Systems 

The essential parts of systems 1 and 2 are described 
in a sequence which traces a typical signal from the 
generator to the projector and from the hydrophone 
to the recorder. 

Test Signal Generators 

The primary signal generators are beat-frequency 
oscillators covering the 15-c to 150-kc range with a re- 
sponse uniform within 0.3 db. A visual indication of 
the frequency setting is provided by a calibrated scale 
on a strip of 35-mm motion picture film 30 feet long, 
coupled through a sprocket chain to the air condenser 
controlling the frequency. The length of this scale in- 
dicates the degree of frequency resolution. The shape 
of the condenser plates is such that the scale gives ade- 
quate frequency resolution throughout the entire 
range. 

A synchronous motor drive provides the lock-in be- 
tween oscillator and recorder when frequency-re- 



Figure 5. \'iew of raft. 


spouse traces are taken, although the dial may be 
operated manually. 

The heterodyne oscillator assembly contains three 
separate circuits. Two of them operate as a beat-fre- 
quency oscillator, one fixed at 650 kc and the other 
variable from 500 to 650 kc. The difference frequency 
of the heterodyned outputs furnishes the signal range 

0 to 150 kc. This arrangement is identical in both sys- 
tems, but the third circuit is fixed at 678 kc in system 

1 and at 747 kc in system 2 for use in tuning the de- 
tector circuit described later in this section. 

Frequency stability has been obtained by design 
features such as (1) mounting the three oscillator cir- 
cuits in the same chassis to have the same ambient 
temperature, (2) separating the component parts with 
networks and buffer amplifiers, and (3) using suitable 
shielding and filters. To correct for the slight drift in 
frequency that still may occur, means for adjusting 
the carrier frequencies are provided and the fre- 
quency scale can be checked by aligning with the 60-c 
power supply and with a 100-kc crystal shunted across 
the oscillator output. Since the adjustment is based 
on the difference frequency, no attempt is made to ob- 
serve the actual frequencies of the carrier oscillators. 

The oscillator furnishes a maximum output level 
of 150 db vs 10—16 watt adjustable over a 40-db range. 
Harmonics in the output voltage are at least 40 db be- 
low the fundamental, and a minimum signal-to-noise 
ratio of 50 db is realized. 

In system 2 a thermal noise signal may be generated 
by using the noise generator in conjunction with the 
heterodyne oscillator. The noise generator includes a 
voltage regulator tube functioning as a wide-band 



72 


USRL TEST STATIONS 


650 --f 



TO PROJECTOR 


FROM HYDROPHONE 


Figure 6. Block diagram of 15 c to 150 kc System 1. 


thermal noise source. Two band-pass filters limit the 
frequency spectrum to 650 ± 0.15 kc or 650 ± 3 kc, as 
desired. These signals are used to replace the fixed 
650-kc signal in the heterodyne oscillator. The hnal 
signal output is then a band of thermal noise 300 c or 
6,000 c wide, centered at the frequency given by the 
oscillator setting. 

The noise output should be at least 10 db below the 
single frequency output to prevent peak clipping in 
the modulator circuit. This adjustment is made by 
controlling the output of the noise generator circuit. 

The midpoint of the noise band in this generator 
is 650 kc for correct frequency alignment of the out- 
put band. The frequency range of the variable oscil- 
lator must be exactly 650 kc to 500 kc. To obtain this 
condition, adjustments are made on the generators as 
follows: The noise generator circuit, functioning 
only as an amplifier, is sharply tuned by a filter 20 c 
wide centered at 650 kc. This is connected between 
the fixed-frequency oscillator and the following mod- 
ulator. The fixed oscillator is then tuned for maxi- 


mum output at any convenient frequency setting. 
With the frequency of the fixed oscillator thus estab- 
lished at 650 kc, the frequency scale is aligned at 60 c 
and 100 kc by adjustments on the variable oscillator. 

Power Amplifiers 

Power amplifiers with a maximum gain of 40 db 
are associated with each system. The maximum undis- 
torted power level of system 1 is 177 db and that of 
system 2, 173 db, but an auxiliary amplifier, described 
later, may be used with either system to reach an un- 
distorted level of 192 db vs 10“^^" watt, or about 1,500 
watts. 

System 1, covering the range from 1 5 c to 150 kc, re- 
quires two output transformers for its power ampli- 
fier. Automatic transfer between them is effected near 
2 kc by a switching circuit operated by a cam on the 
oscillator frequency dial. System 2 employs two power 
amplifiers, covering ranges from 15 c to 30 kc and 
from 300 c to 150 kc. The amplifiers of both systems 
have been designed to operate into a load impedance 



EQUIPMENT AT MOUNTAIN LAKES 


73 


650-f 



TO PROJECTOR 


FROM HYDROPHONE 


Figure 7. Block diagram of 15 c to 150 kc System 2. 


of 135 ohms, which corresponds to the impedance of 
the coaxial transmission lines to the piers. Adjustable 
135-ohm attenuators, designed to carry the full power 
to the amplifiers, are connected in the output circuits 
to supplement the amplifier gain controls, to stabilize 
the output impedance, and to improve the amplifier 
signal-to-noise ratios. The amplifier design and the 
use of output attenuator pads permit a 55-db margin 
between signal and amplifier noise. A bridging net- 
work of 30-db loss at the amplifier output terminals 
allows a 30A transmission measuring set, described 
later in this section, to be used for monitoring 
purposes. 

Transmission Lines 

The 135-ohm insertion loss of the combined trans- 
mitting, receiving, and pier booth cross-connecting 
lines does not exceed 0.25 db at 150 kc, which is too 
small to require any correction for line attenuation. 

Balanced coaxial lines (Figure 11 A) are used for 
signals from power amplifiers to coupling booths on 


the piers, return signals from pier receiving booths, 
for linking transmitting and receiving booths, and for 
interconnecting the various systems. The four-wire 
groups marked “quads” are used for controls such as 
relay operation, intercommunication, and indexing. 

I'he requirements to prevent cross talk are severe, 
as is indicated by the difference in outgoing and in- 
coming levels that may exist. For instance, the hydro- 
phone level in the receiving line may be as low as 
10-16 watt, while the power supplied to a projector 
over the transmitting line may be 150 db above this 
level. In order not to affect the measurements, the 
cross talk must be lower than the received level by at 
least 15 or 20 db, which requires a margin between 
the transmitting and receiving apparatus of nearly 
170 db. In addition to the use of coaxial cable, other 
precautions tending to minimize cross talk have been 
observed, such as a minimum spacing of 6 feet be- 
tween all transmitting and receiving lines, and care- 
ful attention to the laboratory grounding system as 
described later in this section. 


74 


USRL TEST STATIONS 



While the lead-covered coaxial lines provide excel- 
lent transmission, equally good performance for com- 
paratively short runs can be obtained from a twisted 
pair of flexible, single coaxial cables (Figures IIB 
and IIC). The rubber-covered cable is used for lines 
that are exposed to the weather; the cotton-braid 
covered one for inside connections. 

Projector Coupling Equipment 

The projector coupling apparatus is housed near 
the test area in the transmitting booth. The primary 
function of this equipment is to provide suitable im- 
pedances for matching the various test projectors as 
they are connected to the 135-ohm transmitting line. 
Repeating coils provide various sending impedances. 
H-type resistance pads, designed to be used between 
each sending impedance and 135 ohms, permit the 
measurement of available power at any sending im- 
pedance. Two repeating coils are available for han- 
dling power outputs up to 100 watts. One of these has 
seven secondary windings terminating in a multi- 
contact receptacle. The sending impedance is varied 
by inserting in this receptacle one of seven plugs, 
with contacts strapped together in various patterns 
so that impedances of 4, 9, 16, 25, 36, 49, or 64 ohms 
may be provided. The second coil provides imped- 
ances of 135, 600, or 2,400 ohms. A third coil handles 
power outputs up to 1,000 watts at 50, 100, or 500 
ohms, and resistance pads are available for measuring 
the available power at these high outputs. A watt- 
meter circuit for measuring actual power delivered 
to the projector is also available. 

Hydrophone Coupling Equipment 

The hydrophone coupling apparatus is housed on 
the piers in the receiving booths. Its primary function 
is to provide suitable coupling between hydrophones 
and the 135-ohm receiving lines to the laboratory. 

A battery-operated preamplifier of novel design 
provides for either balanced or unbalanced opera- 
tion. A switch in one position sets the input circuit 
for balanced operation. In this case impedance may 
be represented by a shunt resistance of 100 megohms 
and a shunt capacitance of about bfxixi, with a ground 
at the electric center. In the other position the input 
impedance is set for unbalanced operation and may 
be represented as a shunt resistance of 50 megohms 
and a shunt capacitance of about 10/x/xf, with one 
terminal at ground. The ampliher output has been 
designed to feed into the 135-ohm receiving line. The 


Figure 8. 15 c to 150 kc recording system. Bay on left 
shows receiving amplifier and detector. To the right, 
shown in order, are signal and noise generators, recorder 
and thyratron control panel, and power amplifiers. 



EQUIPMENT AT MOUNTAIN LAKES 


Figure 9. Interior of transmitting booth. 

frequency characteristic is essentially flat through 
the range from 15 c to 150 kc and the voltage gains 
for the unbalanced and balanced input conditions 
are approximately +0.5 db and —6.0 db, respectively. 

A battery supply and coupling circuit is provided 
for the frequently used standard hydrophones such 
as the 3A, 5C, and 51) types. A metering panel per- 
mits monitoring of all A and B voltages and currents. 
Switches and jack-terminals j:)rovide for measure- 
ments of various cpiantities, such as response, coup- 
ling, and available power. 1 0 calibrate a hydrophone 
on open circuit requires a knowledge of the loss in 
the coiq^ling circuit. The procedure for determining 
this is to place in series with the hydrophone a re- 
sistance which is very small in comparison with the 
resistance of the instrument. A variable o.scillator of 
low' voltage is applied to this resistor and the signal 
is carried through to the recorder as though from the 
hydrophone itself. After the range of frequencies has 
been covered, the same voltage is connected directly 
to the recorder and the range swept over again. The 
difference between the records in db is the loss in the 
coupling circuit. 

Various types of suj^plementary apparatus are fre- 
(juently required. One such device is a portable bat- 


Figure 10. Interior of receiving booth. 

tery-operated preamplifier that may be placed at the 
edge of the testing area in order to reduce the length 
of the hydrophone cable. Another is an underwater 
preamplifier, operated from the battery for use with 
high-impedance instruments, such as tourmaline 
gauges (tourmaline crystal hydrophones). This am- 
plifier, mounted in a watertight housing, is equipped 
with cable glands for hydrophone leads, battery sup- 
ply connections, and lines for calibration and output 
signals. Several special battery supply, coupling, and 
metering circuits for miscellaneous standard hydro- 
phones are available. Portable low-power d-c supply 
circuits, suitable for various preamplifiers, have been 
designed and are discussed later. 

Rf.ceiving Amplifiers 

In both system 1 and system 2, high-gain, wide- 
band, low-noise-level amj^lifiers are used to increase 
the incoming signal to levels suitable for recording. 
1 he frequency characteristics of these amplifiers are 
flat within 0.2 db from 15 c to 150 kc. 

The coaxial lines are coupled to these amplifiers 
by magnetically and electrically shielded input trans- 
formers, with the input winding balanced to ground, 
riiese recei\ing amplifiers use four low-noise-level 




76 


USRL TEST STATIONS 






2 COPPER 


COTTON 




RUBBER COVERED 
TWISTED 


COPPER 


COTTON 





W.E. CO DUAL COAXIAL CABLE WITH TWO QUADS 


RUBBER COVERED 
COAXIAL CABLE 


COTTON COVERED 

COAXIAL CABLE W.E.CO. 720 CABLE FOUR COAX RUBBER COVERED CABLE 


Figure 11 . Types of coaxial cable used by USRL. 


tubes, heated by regulated direct current, which have 
a maximum gain of approximately +60 db (variable 
in 10-db steps) between the 135-ohm transformer 
input and a 600-ohm cathode-follower output cir- 
cuit. The gain is controllable from —20 db to +60 
db by a split attenuator, comprising two 40-db sec- 
tions, connected at the grids of the first and third 
stages. The attenuation preceding the first stage is 
completely inserted before attenuation of the second 
section is introduced, though both are operated from 
a single shaft. Improved signal-to-amplifier noise 
margin and higher undistorted output levels are ob- 
tained by this method of gain control. 

In the receiving amplifier of system 2 is a second 
attenuator covering 10 db in 1-db steps. There is also 
included a supplementary amplifier, continuously 
adjustable from approximately +20 db to +25 db, 
that is used with the primary receiving amplifier 
when additional gain is required. 

Detectors 

Detector circuits are used with each system for 
obtaining frequency discrimination against back- 
ground and inherent noise, harmonics, and water- 
borne interference, particularly that from the other 
system. 

The general principle of operation is shown in the 
system block diagrams (Figures 6 and 7). The input 


signal / is impressed on the grid of a balanced carrier 
suppression-type modulator through an attenuator 
and a 150-kc low-pass filter circuit. The carrier fre- 
quency, 650 kc — / kc, is brought to this modulator 
from the signal generator through a tuned buffer 
amplifier, controlled by an automatic volume control 
circuit. The buffer amplifier is used primarily to ob- 
tain an adequate margin between the carrier level 
and the maximum signal level in order to minimize 
the unwanted modulation products other than the 
sum frequency. The output of the first modulator is 
then passed through a buffer stage incorporating 
tuned circuits. The tuned circuits pass only the sum 
frequency, (650 — /)+/ = 650 kc, which is impressed 
on the grid of the second modulator. The second 
carrier frequency, from the detector tuning oscillator 
circuit of the signal generator, is brought to this 
modulator through a tuned buffer amplifier, which 
has primarily the function of providing an adequate 
carrier-to-signal level margin. In system 1 the detector 
tuning frequency is 678 kc. The 28-kc difference fre- 
quency from the second modulator is then impressed 
on a crystal filter having an essentially square-top 
pass band of about 12 c centered at 28 kc. The filter 
is followed by one stage of tuned amplification termi- 
nating in 135 ohms. 

The detector circuit of system 2 has three accept- 
ance band widths provided by three crystal filters. 





EQUIPMENT AT MOUNTAIN LAKES 


77 


50K 



Figure 12. Circuit schematic of coupling amplifier. This amplifier provides extremely high input impedances for 
operation in balanced or unbalanced circuits. 


Design considerations in the construction of these 
required the selection of a mid-frequency of 97 kc. 
The detector tuning oscillator of the signal generator 
of system 2, therefore, supplies a frequency of 747 kc. 
A rotary switch permits the rapid selection of any 
of the three band-pass filters, which have widths of 
10, 300, and 6,000 c, centered at 97 kc. Following the 
filter circuits are three stages of amplification termi- 
nating in a 135-ohm output circuit. 

Monitor Converter 

System 2 is provided with a converter circuit com- 
prising a modulator and a local oscillator which may 
be varied continuously from 94 to 100 kc. The prim- 
ary function of this circuit is to permit aural monitor- 
ing of supersonic frequencies by converting the 
normal 97-kc signal output of the detector circuit, 
to an audio frequency range of 0 to 3,000 c. 

Indicator 

The use of narrow-band crystal filters in the de- 
tector circuit makes it necessary to center the output 


of the second modulator precisely on the mid-fre- 
quency. The adjustment of the detector-tuning oscil- 
lator to accomplish this is referred to as “tuning the 
detector” and it is correct when maximum detector 
output is obtained. It has been found expedient to 
provide for a continuous visual indication of this 
adjustment. 

This indication is produced by taking a portion 
of the detector output signal from a constant voltage 
source in the recorder circuit and modulating it with 
a signal from a crystal-controlled oscillator tuned to 
the mid-frequency of the crystal filter in the detector 
circuit. The resultant difference signal is rectified and 
impressed on an electron-ray tube. The shadow angle 
of this tube opens and closes at the difference fre- 
quency. This is a direct indication of the deviation 
in cycles per second from the center frequency of the 
crystal filter. The tuning adjustment may thus be 
maintained within a fraction of a cycle at all times. 
Frequency drifts of the oscillator with respect to the 
filters are minimized by the use of oscillator-stabiliz- 
ing crystals having the same temperature characteris- 


78 


USRL TEST STATIONS 


tics as those in the detector circuit. 

The continuous indication of tuning is essential, 
particularly for frequency response measurements at 
distances greater than about 3 meters. For such a 
length of path through water there is a significant 
delay in transmission. In order that the detector be 
tuned correctly for the incoming signal, it must lag 
behind the oscillator by an amount which is a func- 
tion of the frequency sweep rate and the travel time 
of the sound through the water. The tuning indi- 
cator, which gives a continuous reading, permits com- 
pensating adjustments to be made during the test 
period. 

Linear Level Recorder 

Electromechanical recorders are used with each of 
the systems to provide continuous and permanent 
records of the response of the devices under examina- 
tion. Each recorder consists of an amplifier which 
maintains an arbitrary equilibrium voltage at its out- 
put terminals by controlling, through a motor drive, 
the position of a sliding contactor on a strip attenua- 
tor at the input. A pen attached to this contactor re- 
cords its position on a strip of moving paper. The 
speed of the paper drive is synchronized with the 
frequency sweep of the oscillator, so that the paper 
may have a fixed frequency scale. 

The electronic circuit of the level recorder used 
with system 1 includes a special strip attenuator, a 
second attenuator for presetting the gain, and an 
amplifier tuned to 28 kc, followed by a half-wave rec- 
tifier circuit. The normal d-c output of this circuit is 
about 100 volts at equilibrium and is impressed on 
the grids of a pair of d-c amplifiers. These isolate the 
a-c thyratrons which follow. The anode current of 
each thyratron is passed through one of the windings 
of a small dual armature motor, the field of which is a 
permanent magnet. The thyratron and the d-c am- 
plifier circuits are so arranged that an increase in the 
d-c voltage decreases the normally negative grid 
voltage of one thyratron with respect to its cathode, 
causing it to fire (allow the passage of current) and 
thereby drive the motor in one direction. Conversely, 
a decrease in the d-c voltage produces the same effect 
on the second thyratron, causing it to drive the motor 
in the opposite direction. 

A continuous silk cord, after a few turns around 
the motor shaft, runs over three pulleys and back to 
the shaft. The pulleys are so placed that a section of 
the cord extends the length of the attenuator strip 


and parallel to it. On guides, also parallel, is mounted 
a carriage with an arm, making contact on the strip. 
This carriage is clamped to the cord so the contact 
may be moved to any point on the attenuator by the 
rotation of the motor. 

The recorder seeks at all times to maintain the 
equilibrium d-c voltage at which the thyratrons are 
extinguished, by changing the setting of the contact 
on the input attenuator strip. The maximum rate 
at which the recorder can respond to changes in im- 
pressed level is approximately 100 db per second. 

The resolution of this system is determined by the 
marginal d-c bias on the thyratrons and may be ad- 
justed to within less than 0.1 db. The effective over- 
all stiffness of the electronic and mechanical system 
in the region of balance is determined largely by an 
injected a-c bias, used primarily to control overshoot- 
ing. 

The strip attenuators are wound for a total at- 
tenuation of 50 db at 5 db per inch. They are 
mounted horizontally, directly over the recording 
paper which is a continuous strip with perforations 
along each edge. The paper moves over a roller with 
matching sprocket teeth that is driven by a small 
synchronous motor through an adjustable gear train 
allowing rates of 2, 6, or 18 inches per minute or per 
hour. A friction clutch with a double ratchet attach- 
ment permits the paper to be advanced or rewound 
on the supply spool by means of a hand crank. The 
paper drive motor is tied in to the oscillator drive 
motor so that both may be operated by a single 
switch. 

The frequency resolution of the recorder is a func- 
tion of the frequency sweep rate of the oscillator and 
the speed at which the recording paper travels. Nor- 
mally, the oscillator and recorder are driven at the 
same relative speed (usually the intermediate one) 
to maintain the proper relationship between oscilla- 
tor frequency and the frequency calibration of the 
paper. Under these conditions the individual charts 
for 0 to 150 kc are approximately 32 inches long. 
However, the frequency resolution may be improved 
ninefold by setting the oscillator sweep rate at mini- 
mum and the paper drive at maximum. The system 
is operated most frequently in this manner from 0 
to 460 c with special recording paper. To cover this 
frequency range requires a chart 32.5 inches long. 

The level recorder of system 2 differs from that of 
system 1 chiefly in that its amplifier is designed for a 
flat frequency response from 100 c to 150 kc, and in 


EQUIPMENT AT MOUNTAIN LAKES 


79 



Figure 13. Polar recorder in use with System 2 . 



80 


USRL TEST STATIONS 



Figure 14. Polar recorder turntable assembly with re- 
corder arm raised. 



Figure 15. Magnetic clutch and drive assembly of polar 
recorder turntable. 


the use of a full-wave rectifier, operating on a square 
law characteristic over a level range of some 9 db. 
These points of difference permit the recorder to be 
used for energy measurements of wide-band complex 
wave signals such as noise. 

Polar Level Recorder 

It is often desirable to know the response of an 
instrument for various directions of projection or re- 
ception, and auxiliary apparatus for this purpose is 
provided for both systems. It involves a rotator on 
which the instrument is mounted and a recorder in 
polar coordinates. It is evident that these must rotate 
in exact synchronism for the record to be correctly 
interpreted. Figures 13 and 14 show the turntable as- 
sembly of the polar recorder for system 2, and Figure 
15, the driving mechanism and motor. The turntable 
has suitable positioning and holding devices for 
Sly' X 11-inch sheets of polar coordinate paper. The 
gear train and several electromagnetic clutches allow 
rotation of the turntable in either direction at an 
angular rate of 1, or % rpm, both direction and 
rate being selected by switches. A 5F synchro is 
mounted on the end of the drive shaft, which thus 
couples it to the turntable through a 60: 1 worm gear. 
This synchro is the director of a 5CT synchro at- 
tached to the rotator carrying the acoustic unit being 
tested. Whenever the angular position of this synchro 
does not correspond with that of the director, it gen- 
erates error signals which are impressed on a thyra- 
tron servo amplifier. The altered output of the ampli- 
fier at once modifies the speed of the hp d-c motor 
driving the rotator and thus keeps the turntable and 
rotator in nearly the same angular position. The er- 
ror signal is about 1 volt for each degree of angular 
difference and is thus a measure of the lack of syn- 
chronization which, with proper adjustment, should 
be less than 0.1 of a degree. A hand crank permits 
angular positioning of the turntable independent of 
the motor drive. As shown in Figure 15, the turntable 
may be set readily with the hand wheel, if it is dis- 
connected from the driving motor by the release of 
the clutches. This allows it to be set to any desired 
relation to the rotator, if the circuit between the syn- 
chros is open. The strip attenuator, the sliding con- 
tactor, penholder assembly, and a dual armature 
motor are mounted on a tilt arm pivoted in such a 
manner that it may be lowered into position over the 
turntable (see Figure 14). The strip attenuator has 
been wound at 10 db per inch to a total of 50 db, the 



LQIUPMENT AT MOUNTAIN LAKES 


81 



i 


5CT 

SYNCHRO 


60 ~' • — 
110 V • — 
AC POWER 


THYRATRON 



SERVO 



AMPLIFIER 



ROTATOR 

SHAFT 



Figurf, 16. Schematic of polar recorder servo system. The 5F synchro coiij>lcd to the turntable is the director. The 
thyratron servo amplifier controlled by the 5CT synchro furnishes the power for the l/ 4 -hp rotator drive motor. 


usual range plotted. Plug-terminated patch cords 
are used in establishing connections to the electronic 
circuit and the required power sup})lies. 

Operation of Pulse System 

The pulse system is made up of a pulse generator, 
transmitter modulator, and a receiver modulator and 
pulse rectifier which were designed and built by 
USRL. \Vhen used with the 15-c to 150-kc system, 
acoustic pulses 0.1 to 30 milliseconds in duration may 
be produced and recorded. I'he use of the units is 
illustrated in Figure 17. A continuous single-fre- 
(piency signal is applied to the input of the trans- 
mitter modulator, which acts as a “gating circuit.” 
Fhe output of the transmitter modulator is a pulse, 
that is, a limited train of constant amplitude waves 
of the signal frequency. I'he length and recurrence 
rate of these pidses are controlled by the pulse gen- 



PROJECTOR HYDROPHONE 

Figure 17. Block diagram of System 2 arranged for pulse 
measurements. 


erator. I'hey may be obser\ed and checked on a 
cathode-ray oscilloscoj^e connected across the outj)ut 
of the modulator. After checking, they are amplified 
and applied through the appropriate connection to 
the underwater transducer that is serving as a sound 
source or projector. The nature of the resulting 
acoustic signal depends, of course, on the electro- 
acoustic properties of the transducer. 

The acoustic signal generates in the hydrophone 
an electric signal that is amplified and applied to the 
linear or polar recorder attenuator as desired. After 
further amplification, the signal is passed through 
the receiver modidator, which is another gating cir- 
cuit similar to the transmitter modulator. The receiv- 
ing time can be controlled by the pulse generator in 
such a manner that any portion of the received signal 
may be accepted for measurement and the rest re- 
jected. To aid in this adjustment, a cathode-ray oscil- 
loscope is used to observe the incoming signal after it 
has passed through the receiver modulator. A switch 
])ermits the direct comparison of the total signal with 
the portion accepted for measurement. This plan 
allows the elimination of reflections which would be 
j)resent in continuous-wave measurements and would 
result in an erroneous signal level. 

If the pulses occur at the rate of 15 per second or 
more, the pulse rectifier produces a d-c voltage that 
is suitable for controlling the recorder circuit. 

Units of the Pulsing System 

Pulse (jeiierator. I'he generator produces the 
pulses governing the action of the transmitter and 
receiver modulators. It consists of three unbal- 
anced multivil)rators. A, B, and C, that will produce 


82 


USRL TEST STATIONS 


negative rectangular pulses when triggered. In addi- 
tion, there is a relaxation oscillator capable of being 
synchronized with various subharinonics of the 60-c 
filament supply. Short, sharj), positive impulses from 
this oscillator are used to trigger stages A and B 
simultaneously. The rectangular pulse from A con- 
trols the transmitter modulator, and its length de- 
termines the length of the modulator signal. 

The negative rectangular pulse from B is differen- 
tiated, yielding a sharp negative impulse at the begin- 
ning and a sharp positive impulse at the end. Multi- 
vibrator C is triggered by the latter (C responds 
only to positive impulses) after A and B are triggered 
and at a time determined by the length of the rec- 
tangular pulse from stage B. The rectangular pulse 
now generated in stage C is used to control the active 
receiving time of the receiver modidator. 

Idle recurrence rate of this secpience can be set at 
60, 30, 15, or 3 times per second by means of a selec- 
tor switch. The pulse length of the multivibrators is 
controlled by the time constants of the associated 
resistance-capacitance [RC] circuits. Each stage has 
two such controls. A calibrated smooth change of 
resistance covers a time ratio of 20, and three fixed 
condensers give three decades of pulse length. \Vith 
this arrangement it is possible to cover pulse dura- 
tions from 0.1 to 30 milliseconds with overlapping 
scales for the whole range. 

Trausinitter Modidator. The transmitter modula- 
tor is essentially a stage of push-pull amplification 
with a cathode resistor which serves also as the cath- 
ode resistor of a 6L6 tube. The voltage drop across 
this resistor, due to the current drain of the 6L6, is 
made sufficient to bias the amplifying tubes of the 
push-pull stage beyond cutoff and thus keep them 
from passing any signal. The surge from the genera- 
tor is amplified, and the resulting large negative pulse 
is applied to the grid of the 6L6, which stops con- 
ducting and allows the push-pull amplification to act 
normally for the pulse period. An output transformer 
is used with this push-pull stage in order to eliminate 
the d-c components owing to the amplifying tubes 
j:)assing from a nonconducting to a conducting state 
and back again during the pulsing sequence. These 
components may be observed on a cathode-ray oscillo- 
scope when there is no signal frecpiency being ampli- 
fied. They are balanced by adjusting the screen-grid 
potentials of the am|3lifying tubes. An output trans- 
former is used that has an essentially flat frecpiency 
characteristic from 1 to 150 kc. 


I he transmitter modulator has input and output 
impedances of 1 35 ohms. It operates from a d-c B sup- 
ply of 275 volts and an a-c filament supply of 6.3 volts. 
The gain of the unit is 10 db and the maximum un- 
distcn ted power output is 145 db vs lO-i^ watt. The 
power output between pulses is more than 70 db 
below the maximum undistorted pulse output. The 
transients due to imperfect d-c balance are 50 db be- 
low the same maximum. 

Receiver Modidator and Pulse Rectifier. The op- 
eration of the modulatcjr section of the receiver 
modulator and pulse rectifier unit is very similar to 
that of the transmitter modulator but the operational 
characteristics are different. It has a high input im- 
pedance designed to work with the amplifier of the 
recorder circuit. It is capable of discriminating 
against the highest signal output of the cathode-fol- 
Icjwer stage in the preceding amplifier. Hence, any 
portion of the incoming signal may be selected with- 
out interference from the rest of the signal. This se- 
lection is controlled by adjustments on the pulse 
generator. 

The conversion of the recurrent pulses from the 
modulator into a d-c voltage suitable for operating 
the power level recorder is not simple. This voltage 
produced must satisfy two requirements: 

1. Its a-c component must be smaller than the 
change in the d-c voltage inherent in the resolution 
of the recorder. In other words, its magnitude will 
determine the resolution obtainable without appre- 
ciable instability, though the final limit is set by the 
nature of the recorder circuit. 

2. It must be capable of changing about its equilib- 
rium value at least as fast as the pen-drive motor can 
change the level of the signal into the pulse rectifier. 
If this condition is not met the recorder system will 
hunt, though this may always be avoided by decreas- 
ing the motor speed. 

The circuit producing the voltage which meets 
these requirements is shown schematically in Figure 
18. The operation is as follows: The receiver modu- 
lator is adjusted to pass a short pulse (0.3 to 0.6 milli- 
second) of the acoustic signal to be measured. This 
input to the pulse rectifier at point a and the re- 
sultant rectified voltage at point b are indicated on 
the drawing. The condenser C is made small (0.005 
/xf) in order that it may be charged to full value with- 
in the duration of the pulse. The resistor R is chosen 
so that 1 / RC is approximately ecpial to the pulse 
repetition frecpiency. 1 his allows the condenser to 


EQUIPMENT AT MOUNTAIN LAKES 


83 



Figure 18. Circuit schematic of the pulse rectifier. The 
voltage wave form of the pulse is indicated at various 
points in the circuit. 


A gain control at the input of the receiver modula- 
tor allows the sensitivity of the pulse recording sys- 
tem to be made ecjual to that of the usual continuous- 
wave system. However, the sensitivity of the pulse 
system is somewhat dependent on the length of the 
received pulse and the repetition rate. In nearly all 
tests, however, the values of these variables will be 
chosen and held constant throughout. 

7 he frequency response of the receiver modulator 
and pulse rectifier is flat within ±0.5 db from 1 to 120 
kc. The response at the low-frequency end is con- 
trolled largely by the number of cycles of the signal 
within the pulse being measured. 

Miscellaneous Features. Auxiliary Apparatus 


become almost wholly discharged between pulses. 
Hence, the grid signal is a saw-tooth wave, the am- 
plitude of which can change rapidly with change in 
the incoming pulse. 

The first section of the 6SN7 acts as an impedance 
changer and phase inverter, the voltage at point c 
still containing the d-c and a-c components of the 
rectified signal at b. The a-c component of this volt- 
age is applied to the grid of the second section of the 
6SN7 through the RC network. The time constant of 
this RC combination should be approximately equal 
to that of the filter section. By proper adjustment, 
the signal at d can be made equal to the a-c compon- 
ent of the signal at c, but inverted in phase so that 
the mixed voltage at e will be approximately equal 
to one-half the d-c component of the voltage at c 
with the a-c components balanced out. To facilitate 
this adjustment, a terminal is supplied for observa- 
tion with a cathode-ray oscilloscope. 

An increase in the intensity of the received acoustic 
signal causes the d-c voltage at e to rise. The thyra- 
tron control circuit used with the pulse system re- 
quires, however, a decrease in the d-c voltage with in- 
crease in signal intensity and, for this reason, the 
final tube shown is used. 

In order to be used with the pulse system, one of 
the thyratron control circuits is modified and a single 
coaxial jack installed to take the d-c output of the 
pulse rectifier. A switch on the front of the panel 
allows the operator to choose the output of either 
the recorder circuit or the pulse rectifier. When the 
latter is chosen, 50-ohm resistors are automatically 
inserted to slow the pen-drive so that the recorder 
will not hunt. This reduces the speed of the recorder 
from 100 to some 30 db per second. 


Methods of Connecting. Great flexibility of inter- 
connection is obtained by the use of jack fields as 
terminals for the individual pieces of electric ap- 
paratus mounted in the bays. The arrangement of 
the jacks within the field is based on factors such as 
accessibility, convenience in wiring, and the consid- 
eration of cross talk. Many jacks are interconnected 
so that commonly used combinations of apparatus 
are established without the use of external connec- 
tors. The jack fields also provide terminals for inter- 
bay, inter-system, and system-to-pier lines, and for 
frequently used coils, attenuator pads, and load re- 
sistors. 

Connections between jacks are made with plug- 
terminated, flexible, shielded cords referred to as 
patch cords. Where the jack grouping permits, con- 
nection between adjacent jacks is made with short- 
circuited cordless plugs. The types of patch cords 
and plugs may be seen in Figures 9 and 10. 

Grounding. Each system is provided with a funda- 
mental circuit ground comprising a copper pipe 
driven into the lake bottom adjacent to the pier test- 
ing area. Four No. 0000 stranded copper cables con- 
nect each fundamental ground to heavy copper bus 
bars in the pier booths and apparatus bays, and to 
copper strips mounted along the test areas which 
are used for grounding test apparatus. Individual 
circuit ground connections are made directly from 
the bus bars to all panel-mounted equipment. The 
lead sheath and outer conductors of the coaxial trans- 
mission lines between the pier booths and the labora- 
tory are grounded at the jacks in the pier booths. The 
types of ground are indicated on the jack fields by 
means of colored celluloid windows placed over the 
designation strips. 




84 


USRL TEST STATIONS 





Figure 19. 30A iransniission measuring set connected in 
system jack field. 


Transmission Measuring Set. The standard instru- 
ment adopted for power level measurements is a 
Western Electric 30A transmission measuring set. 
The essential elements are a thermocouple, indicat- 
ing meter, attenuators, and switching circuits. The 
input impedance is 135 ohms and the set operates at 
frequencies up to 150 kc. The readings may be varied 
in increments of 1 db through a range of 90 db by 
means of attenuator sections connected by dials and 
switches. A high degree of convenience has been ob- 
tained by the use of jack terminations for the individ- 
ual circuit elements. The circuit of the instrument 
provides for both gain and loss comparison paths, 
selected by a switching key. 

The meter scale covers a range from — 10 to +3 db 
on either side of a center zero. The design is such that 
the zero reading means a power level of 130 db vs 
10~i® watt (1 milliwatt). An internal d-c circuit pro- 
vides for the maintenance of the thermocouple and 
metering circuit calibration. Recalibration of the 
metering circuit may be necessitated by thermocou- 
ple aging, temperature changes, and thermocouple 
replacements. The overall accuracy of the transmis- 
sion measuring set is ±0.1 db. 


Monitor Amplifiers and Speakers. Each system is 
provided with means for listening to the received 
signal. System 1 has an audio-amplifier with a maxi-* 
mum gain of about 60 db and an output power of 
some 12 watts. This amplifier is used in conjunction 
with a dynamic speaker. System 2 is equipped with 
a monitor converter circuit that allows the use of 
earphones at the output of the detector circuit. The 
monitor output signal may also be sent through per- 
manently installed lines to the power amplifier and 
loud-speaker of the reproducer set. 

Reproducer Set. A transcriber is provided for re- 
producing, electrically and acoustically, for calibra- 
tion purposes, various types of water noises includ- 
ing a number of ship noises. The transcriber has two 
separate turntables, each with a reproducer for verti- 
cal or lateral cut records at 33^ or 78 rpm. A pream- 
plifier associated with each head permits individual 
control of level and of frequency weighting charac- 
teristics. A 50-watt power amplifier is used for the 
operation of a high-quality speaker. The frequency 
response of the amplifier without equalization is sub- 
stantially flat from 30 to 10,000 c. 

High-Pass and Low-Pass Filters. A number of high- 
and low-pass 600-ohm filters have been assembled 
and connected through suitable switches so that 
single units or combinations may be used. The high- 
pass hlters have cutoff frequencies of 0.2, 0.7, 2, 15, 
33, and 60 kc; the low-pass filters, 0.7, 2, 5, 15, 35, 70, 
and 150 kc. All filters have attenuation beyond the 
cutoff frequencies of more than 50 db. 

Laboratory Intercommunication Systeins. Each test 
system has microphone-speaker communication be- 
tween the laboratory and the piers. It has been ar- 
ranged so that it is j)ossible to contact each pier from 
any one of the systems and vice versa. This intercom- 
munication is required to permit proper coordina- 
tion between the operators at separate locations 
while making calibration measurements. 

Mechanical Features of Outdoor Facilities 

Handling Facilities. Overhead monorails are pro- 
vided for trolleys to which chain hoists are attached. 
Rail switches permit the placing of hoists carrying 
rigged apparatus on storage spurs or in any arrange- 
ment desired for testing. The hoists have capacities 
up to 1 ton, and the rail height permits a lift above 
the piers of 16 feet. 

Suspension Members. Standard laboratory hydro- 
phones and small test devices are usually rigged on 



EQUIPMENT AT MOUNTAIN LAKES 


85 




Figurk 20. I')pical hydrophone mountings: (A) lA hy- 
drophone, (B) 3.\ hydrophone, (C) XMX hydrophone. 


three-piece extension rods. I'lie upper section may 
be adjusted over some 2 feet by means of a crank- 
operated screw. 1 he lower section is also adjnstal)le, 
but only in steps. Center sections of various lengths 
may be inserted between these two ends. I'he upper 
section has a .scpiare mounting plate set in gimbals 
which permits the rod to hang vertically, if symme- 
trically loaded. I'his plate rests on a shock-absorbing 
receptacle and allows an assembly to face in any one 
of the four directions. The lower section terminates 
in a short crossbar that will accommodate various 
coupling fixtures. Figure 20 shows a typical mount- 
ing. 

Devices u]j to 1,500 pounds are suspended from 1- 
inch extra-heavy pipe available in convenient lengths. 
One end has a heavy-duty eye for the hoist hooks and 
the other has standard pipe thread for attachments 
such as flanges, yokes, F bars, and brackets. Figure 
21 shows the more common fittings. Pipe is the usual 
support for standard projectors rigged for semiper- 
manent service and for those which are not rotated 
for pattern studies. It is necessary to keep lengths that 
are not in irse hung vertically to avoid bending. 

Clamping block assemblies, designed for securing 
the pipes at the retjuired positions, are shown in Fig- 
ure 22. Convenient features include: rapid locking 
and release without .separate wrenches, free angular 
motion of 40 degrees between the block and its base 



Figurk 21. Fransclucer suspensioii pipes. 


to orient the transducers, interchangeable antishock 
mountings to suit the transducer loading, leveling 
screws and built-in levels for trimming the entire 
assembly to the vertical. 

Rotator. All test devices which require continuous 
rotation for observation of beam patterns are sus- 
pended from a rotator mechanism. The assembly may 
be seen in Figures 23 and 24. The rotator has two con- 
centric drive shafts that rotate either individually or 
jointly in synchronism with the polar recorder turn- 
table previously described in this section. Independ- 
ent rotation of the concentric shafts is required for 
studies of transducer-dome assemblies. For other 
tests, a single shaft is sufficient. 

1 he housing of the rotator contains the driving 
gears, radial and end-thrust bearings, and the me- 
chanical clutch assembly. Each shaft has a clutch to 



Fk;urk 22. Special clamping blocks for holding suspen- 
sion pipes on turret type carriage. 



86 


USRL TEST STATIONS 



Figure 23. Rotator and suspension framework. 


Figure 24. Placing rotator in position in carriage. 



engage the driving motor, and a control for each 
projects through the housing. Each shaft has a 5CT 
synchro coupled to it through a 60 : 1 gear train. The 
synchros are mounted in an auxiliary housing on the 
side opposite the clutch controls. The driving motor 
rests on top of the gear box and is coupled to the 
internal gear train through an assembly that pro- 
vides for manual operation by a crank. 

The concentric shafts are approximately 3 feet 
long. The outer one, a 3-inch tube, terminates in a 
metal framework, adjustable in length. The inner 
one is extended by a 1-inch pipe of adjustable length, 
which is terminated in a 4-inch flange, and is centered 
with the bottom plate of the outer shaft assembly. 
7 he approximate load capacity of the rotator is 1,000 
pounds per shaft. 

Mounting Fixtures. Universal mounting fixtures 
and others of special application are used for attach- 
ing test transducers to the various suspension mem- 
bers. Examples of these fixtures are shown in Figures 
20A and 20B, which illustrate tlic devices for lA and 
3 A hydrophones. Figure 20C shows a fixture used for 
rigging a variety of small and medium size trans- 
ducers. 


Carriages. The carriages that are used to support 
the suspension members roll across the test areas on 
flanged wheels fitted with brakes and matched to the 
side rails. An H-type carriage, shown in the fore- 
ground of Figure 25, is used for supporting trans- 
ducers that do not require rotation. A turret type is 
shown in Figures 22 and 26. The upper assembly may 
be rotated at % rpm by a small synchronous motor 
acting through reducing gears and a rubber friction 
wheel. Studs attached to the underside at 30-degree 
intervals operate a microswitch to indicate at a re- 
mote point the angular position of the turret. How- 
ever, because of the superior facilities provided by the 
rotator, the turret type is now used only for medium 
and heavyweight instruments not requiring steady 
rotation. 

A special carriage is used for the rotator assembly 
with the housing resting on a flat bedplate. A split 
radial thrust bearing on the underside engages the 
3-inch outer shaft and adds to the rigidity of the sys- 
tem. Two lever-operated brakes .secure ihe carriage 
at any position. 

Screens. Screens are used frequently to minimize 
standing waves resulting from surface and bottom 



EQUIPMENT AT MOUNTAIN LAKES 


87 




Figure 25. Determining the testing distance between 
transducers. Rotator shown at far end of test area is 
mounted on “H” type carriage. 


reflections.'^ The most common screens are thin- 
walled watertight metal envelopes about 2 by 4 feet 
by 1 inch containing a sheet of hard felt. Flanges 
along the 2-foot edges permit the assembly of several 
screens in multiple-unit configurations. Figure 27 
shows a V assembly being placed in position to func- 
tion as a surface screen, that is, to reduce surface 
reflections from the region between a projector and 
a hydrophone. Similar assemblies are sometimes 
placed below the acoustic axis of transmission with 
the V inverted to minimize bottom reflections. 

Special Facilities. Compressed air is generally 
available from an outlet and special filter in the 
transmission booth of pier 1. This installation is 
primarily for charging the reservoirs of 4A- and 4B- 
type low-frequency projectors. Other general uses 
include cleaning and drying the less accessible parts 
of miscellaneous gear with air blasts. 

A portable gear pump delivering about 10 gallons 
per minute and driven by a reversible motor is pro- 
vided for use on the piers. A long connecting shaft 
allows the pump to be immersed in the lake, thus 
providing a water supply free from air. The pump is 
used for washing and debubbling test transducers 
and for filling and emptying domes not provided 
with drain plugs. 

An underwater lamp and a viewing device are 
available for examining transducers and their rig- 
ging in test positions. 


b A general discussion of screens, from a theoretical stand- 
point, is given in Chapter 4. 


Figure 26. Typical test arrangement using turret type 
carriage. Portable coupling amplifier in foreground. 

A portable box of wrenches, screwdrivers, pliers, 
and other tools used for rigging is maintained at each 
pier. A complete supply of rules, graduated steel 
tapes, levels, and general marine hardware is avail- 
able for measurements and rigging. 

Maintenance of Performance 

Safety Precautions. Units having dangerously high 
voltages are equipped with safety switches which 
must be opened before access to the unit is made. 
This is accomplished by an interlocking mechanism, 
which must be checked frequently for correct opera- 
tion. 


Figure 27. Lowering the “V” screen into position for re- 
ducing surface reflections. 




88 


USRL TEST STATIONS 


No one is allowed to work on a pier alone when 
weather conditions are hazardous. 

Electrical. The power is turned on about 3 hours 
before any measurements are made so that thermal 
equilibrium is reached to minimize any drift in fre- 
quency or level. Check observations are always made 
at the beginning of each day and, if necessary, re- 
peated at intervals. The performance of each regu- 
lated rectiher is checked by means of a built-in 
metering circuit, which permits the observation of 
output voltage and load current distribution among 
various tubes. This is required frequently as faulty 
tubes result in a nonuniform load distribution. 

The calibration of the 30A transmission measuring 
set is described earlier in this section. 

The signal generator is adjusted for continuous- 
wave, noise, pulse, or other types of tests. 

The wide-band receiving amplifier, detector, and 
recorder are checked as a unit. Individual circuits are 
tested only when the unit fails. To observe the over- 
all performance and adjust the sensitivity, the oscil- 
lator is set at 10 kc and adjusted to 130 db vs 10 
watt with the 30A set. The signal is fed through the 
wide-band amplifier and the detector with zero gain, 
and the recorder is adjusted to the same level entering 
the amplifier. 

The resolution, stiffness, and over-shoot of the re- 
corder are adjusted and the rate of response to chang- 
ing levels observed. Any substance that increases the 
friction between the sliding surfaces reduces the re- 
sponse rate and may soon cause serious wear. 

The carrier frequency can be balanced out at the 
first modulator of the detector circuit with the signal 
generator set at 0 c. The controls for balancing the 
rejection circuit are adjusted until the recorder sig- 
nal is at a minimum. 

In addition to these individual adjustments, it is 
desirable to determine the overall gain of the test 
system. The circuit and the levels used are shown in 
Figure 28. A level of 160 db vs watt is estab- 

lished at the projector side of the impedance match- 
ing coil with the 30A set connected to this coil 
through a 20-db matching pad. The oscillator and 
power amplifier gain controls are adjusted until the 
correct power level is indicated on the set. Then the 
oscillator frequency dial and recorder paper scale are 
synchronized at 0 c and a response is taken up to 150 
kc. Since irregularities in the performance of any of 
the component parts will show up in the record, the 
system is considered to be in operating condition if 



0 08 CAIN adjustment • (.EVEL IS GREATER THAN INDICATED Br THE 

UJSS IN THE IMPEDANCE MATCHING COH. 


Figure 28. Arrangement for overall system check. Signal 

levels are given in decibels versus 10'^“ watt. 

the recorder curve is satisfactory in magnitude and 
flatness. 

The test systems are completely overhauled after 
a year’s use. All tubes are tested; patch cords are 
checked for continuity and leakage; coil and attenua- 
tor characteristics are measured. Transmission and 
leakage characteristics of all lines are checked. Poor 
leakage-to-ground in coaxial lines may result from 
foreign particles, and, if so, is usually corrected by 
discharging a 500-/xf condenser at 200 volts over the 
path. After reassembly, individual circuits are 
checked and adjusted, if necessary. 

Mechanical. It has been found advisable to estab- 
lish certain routines in the maintenance of the me- 
chanical equipment. Important groups of these are 
the following: 

1. All electric contacts including plugs and jacks 
are cleaned at regular intervals to forestall troubles 
arising from dirt and corrosion. Rough or damaged 
surfaces are refinished or replaced. 

2. The piers are examined periodically for settling 
by placing levels both on and across the rails, which 
were designed with bolts for adjustment. 

3. The test equipment, especially that used out- 
doors, is protected from the weather as much as pos- 
sible. This includes covering when not in use, keeping 
pier booths and windows closed, and draining or 
providing underwater storage for free-flooding de- 
vices. 

4. All mechanical equipment, such as chain hoists, 
rigging fixtures, etc., is regularly inspected, lubri- 
cated, and stored under shelter during the winter. 

6.2.2 Practical Calibration Procedure 

Comparison Tests 

A large part of the calibration work done by 
USRL, particularly in the range from 15 c to 150 kc, 


EQUIPMENT AT MOUNTAIN LAKES 


89 


was comparison ot instruments in a free field. Refer- 
ence has been made in Chapter 5 to the advantages of 
this method, and it seems probable that it will con- 
tinue to be an important one. The following discus- 
sion of testing procedures pertains to transducers in 
general, the distinction between hydrophones and 
projectors being made only when the nature of the 
device affects the techniques. 

Required Background Information. In planning 
an efficient program, it has been found expedient to 
know in advance the size, shape, and other character- 
istics of each device. The most essential information 
is its actual performance. However, knowledge of the 
end application and of the operating principle per- 
mits emphasis tp.be placed on the characteristics of 
primary importance. The following illustrates the 
type of information to be included: 

1. Physical Characteristics 

Size of diaphragm 
Location of diaphragm center 
Location of center of rotation 
Configuration and type of active elements 
Position in use with and without auxiliary 
gear 

Drawings showing dimensions and mount- 
ing details 

Temperature and pressure limitations 

2. Electrical Characteristics 

Terminal impedances 
Direct-current power requirements 
\V^orking and maximum power input for con- 
tinuous-wave and for pulse operation 
Tuning and associated network requirements 
Circuit schematics 
Frequency range 

Ordinarily there are points of particular interest 
in each program. For example, it may be desired that 
special attention be given to a restricted portion of 
the frequency range, directivity patterns be taken 
about certain axes or at specific frequencies, a de- 
tailed investigation be made of the secondary or ter- 
tiary resonanass of a sharply resonant device, or the 
performance of a projector be studied at specific 
power inputs. 

Preparation for the tests may be expedited if all 
this information is furnished well in advance of the 
testing date. A person thoroughly familiar with the 
instrument under observation can be of material as- 


sistance in the testing program, and one should be 
present if possible. 

Rigging. Throughout the rigging, every precau- 
tion is taken to protect the instrument from mechani- 
cal shock or other injury, and crystal devices that are 
injured by high temperatures should be shielded 
from the sun as much as possible. In many cases a 
large portion of the time is devoted to rigging the 
test devices and associated gear. Each device presents 
different rigging problems arising from disparities in 
weight, size, frequency range, and the type of tests 
planned. There are, however, many rigging consid- 
erations common to most testing programs, and these 
will be considered in the order in which they arise. 

The types of suspension for positioning transducers 
are described in Section 6.2.1. The selection of a par- 
ticular one is dependent on the physical character- 
istics of the instrument as well as on the nature of the 
observations to be made. In rigging for tests in which 
the response is wanted only at a few angular positions, 
a heavy instrument is mounted on 1-inch pipe, while 
a light one usually is mounted on a hydrophone rod. 
Since the rod is supported by gimbals and an anti- 
shock mounting, it requires symmetrical loading. In 
cases where the instrument construction does not 
allow this loading, the 1-inch pipe may be used or 
the device may be mounted on the rotator. When the 
pipe is used, it is clamped in position by blocks on 
rubber mountings. This protects against shock but 
does not provide a completely free suspension. In any 
case, care must be taken to have the whole assembly 
hang in a truly vertical line, particularly if the dis- 
tance between instruments is measured after they are 
submerged. The test distance is taken to be that be- 
tween the upper parts of the suspension rods, with 
corrections for the position of the transducers rela- 
tive to these rods. Where the testing distance is short, 
small deviations from the vertical may result in ap- 
preciable errors. Such irregularities in hanging may 
also displace the instrument with respect to the acous- 
tic axis of the standard instrument, introducing seri- 
ous errors at frequencies where sharp beam patterns 
occur. 

In rigging a test device to a pipe suspension, an at- 
tempt is made to have the center of gravity of the 
instrument lie on the major axis of the support and 
the plane of its active face parallel to the axis. Adjust- 
ment screws and small levels in the mounting blocks 
facilitate the process, and counterweights may also be 
employed. If the required adjustment is not too large. 


90 


USRL TEST STATIONS 


the assembly may be leveled after immersion. The 
mounting of even the heaviest instruments on the 
rotator does not require leveling because of its ri- 
gidity. 

Determination of Test Conditions. Before an in- 
strument is finally positioned, the depth, testing dis- 
tance, and electric coupling are determined in as 
close accord as possible with the principles set forth 
in Chapter 5. It is evident that the final setup will 
often be a compromise with ideal conditions. 

The depth of the testing areas is about 6 meters, 
though most instruments are tested at 2 to 3 meters. 
The exact depth may be selected with the hydro- 
phone rod and the pipe suspensions. The length of 
the hydrophone rod may be set approximately before 
immersion, and adjusted afterward by the lead screw. 
With the rotator, the adjustment of depth can only 
be made in rather large steps and with no change pos- 
sible after immersion. This requires that the associ- 
ated testing instruments be adjusted to operate with 
it, whatever its position. 

Estimates of testing distance based on instrument 
size and frequency range are usually made before the 
tests, so that the pier location of the instruments may 
be tentatively determined. Several testing distances 
are commonly used to observe the effects of the bot- 
tom and surface reflections and of standing waves be- 
tween the instruments. Projectors are usually faced 
away from the shore, to avoid first-order shore reflec- 
tions. 

When several projectors are mounted to calibrate 
a sound field over a wide frequency range, the higher 
frequency ones are mounted near the shore and di- 
rected toward open water, while the lower frequency 
devices are suspended at the far end of the pier and 
directed toward shore. The test hydrophone is 
mounted on the turret between the two projector 
assemblies. This arrangement permits the hydro- 
phone to be positioned with respect to either pro- 
jector. An alternative method involves mounting the 
two projectors back to back in the turret so that 
either may be quickly set into proper relation to the 
fixed hydrophone. Such an arrangement may be seen 
in Figure 22. 

Usually the recommended electrical conditions are 
approximated as closely as possible. If the device is a 
projector, the recommended source impedance can 
usually be matched with an available coil. If it is a 
hydrophone, it may be connected directly to a high- 
impedance coupling amplifier or terminated in ac- 


cordance with the recommended operating con- 
ditions. 

Final Preparation for Testing. All too often the 
leads furnished are too short to permit testing at the 
proper depth. A number of single coaxial cables are 
available for extending such leads. Splices are made 
watertight by several layers of rubber tape or by the 
use of underwater junction boxes. The cable is then 
taped to the supporting rod and, in the case of heavy 
cable, wound about the rod to prevent asymmetrical 
loading. 

Before any instrument is tested, it is thoroughly 
washed and debubbled, since significant errors may 
be introduced by air bubbles or films. The active sur- 
faces are washed with a soft cloth soaked in a strong 
soap solution to which a generous supply of wetting 
agent has been added. This procedure removes oil, 
grease, or dirt particles which occlude air. The man- 
ner in which the water meniscus traverses the instru- 
ment face when it is lowered and raised in the water 
is a good criterion of cleanliness. The meniscus pro- 
gresses smoothly and without breaks if the face is 
thoroughly wetted. A device having structural irregu- 
larities which may trap air on, or near, the face is care- 
fully debubbled after it is submerged by an air-free 
stream of water from an underwater pump or hand 
syringe. 

After an instrument has been rigged and thor- 
oughly cleaned, it may still require soaking. This is 
necessary to reach thermal equilibrium for x-cut Ro- 
chelle salt crystals or others of high thermal inertia. 
Such instruments are suspended in the water for sev- 
eral hours before tests or even overnight, depending 
on the size of the device and the difference between 
air and water temperatures. 

The resistance of the device and the insulation 
resistance between terminals and case are checked 
before submersion and during tests if leakage is 
suspected. 

After the instruments are positioned and electric 
connections made, preliminary trials are made to de- 
tect extraneous noise, cross talk, or signs of overload- 
ing the hydrophone or receiving equipment. Exces- 
sive noise is easily heard. Many test devices used in salt 
water are not provided with electric shielding, and 
so pick up power-line frequency from ground cur- 
rents. When grounding adjustments do not improve 
the signal-to-noise ratio sufficiently, it may be neces- 
sary to insert suitable rejection filters ahead of the 
wide-band receiving amplifier to prevent overload- 


EQUIPMENT AT MOUNTAIN LAKES 


91 


LOG 


Mr,L,_ _TEST STATION ^SYSTEM; __/_PIER 

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Figure 29. Sample log sheet. 


ing by the noise. Overloading of the preamplifier as- 
sociated with the test hydrophone should be investi- 
gated, particularly in the case of devices having a high 
sensitivity. Overloading may be detected with a 
cathode-ray oscilloscope which shows a nonlinear re- 
lation between the input pressure and the output 
signal. The sound pressure on the hydrophone may 
be reduced by decreasing the driving power of the 
projector or by increasing the test distance. 

Cross-talk levels may be investigated by spacing the 
projector and hydrophone about 3 meters and by ad- 
justing the system gain controls to a high recorder 
level at some convenient frequency. A rapid change 
in frequency, effected by manual operation of the 
dial, should show an abrupt drop in the output signal 
because of the effective detuning action of the detec- 
tor. The magnitude of this change is a direct measure 
of the margin between the acoustic signal and the 
electric interference. 


Test Observations. The recorder charts of runs for 
acoustical and electrical data are supplemented by de- 
tailed log and circuit sheets giving an index to the 
series of tests, identifying the various runs, and noting 
the instruments used and the circuit arrangements. 
To facilitate the recording and analysis of the data, 
each run is numbered according to the test sequence, 
and each chart is given an identifying letter. Other 
entries on the log sheets include the water tempera- 
ture, testing depth, pier positions of the instruments, 
and the time at which each run is taken. Sample log 
and circuit sheets are shown in Figures 29, 30, and 31. 
In addition to the data, descriptions of the test de- 
vices are included in the form of blueprints and 
schematics furnished by the maker of the apparatus, 
rough sketches made at the laboratory, or photo- 
graphs. 

Test observations are usually made to calibrate a 
sound field with a standard hydrophone, to calibrate 


92 


USRL TEST STATIONS 


CIRCUITS 

tlr.L j^Test Station 

Project Nn. OOP Datp l/~ /S'- ^ ^<~>py Sheet No. I \— 

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Ei(;ijrf. 30. Sample circuit sheet No. l-l. 



EQUIPMENT AT MOUNTAIN LAKES 


93 


CIRCUITS 

Test Station 


Project No Date Copies,^ Copy to^^^_Sheet No. T ^ 

PeFtr^irV'O^ QCjJj ^5/7~/Oa^ ■»* -d" 

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Figure 31. Sample circuit sheet No. 1-2. 



94 


USRL TEST STATIONS 


transducers functioning primarily as hydrophones, 
and to calibrate transducers designed to function pri- 
marily as projectors. Reversible transducers are cali- 
brated functioning in each capacity. 

Reference data from the first procedure are re- 
quired only when the test transducer is to be cali- 
brated as a hydrophone. These runs generally pre- 
cede observations on the test instruments and are re- 
peated, in part at least, once each day during the test 
period. 

Reference Data. When a device is to be calibrated 
as a hydrophone, one or more standard projectors are 
selected to cover the frequency range of the test in- 
strument and the sound field established by each of 
them is calibrated by at least two standard hydro- 
phones. One of the hydrophones should have direc- 
tional characteristics which will discriminate against 
surface and bottom reflections, though reflective ef- 
fects may be decreased by operating at short distances 
within the limits discussed in Chapter 5. Whenever 
certain frequencies are of particular interest, as in the 
case of sharply resonant devices, it is advisable to 
choose standard hydrophones having minimum ir- 
regularities of response at these frequencies. 

Before reference runs are made, the angular posi- 
tion of each projector is set where the maximum 
acoustic output appears on the recorder. This train- 
ing is done at a frequency where the beam pattern of 
the projector is sharp, and at a distance sufficient to 
minimize the effect of standing wave patterns be- 
tween transducers. With low-frequency projectors, 
the beams are usually broad enough so that careful 
mechanical alignment in rigging is adequate. Once 
adjusted, each projector remains undisturbed 
throughout the tests. 

Runs are taken at several distances and agreement 
between the computed and observed distance losses 
indicates the absence of significant standing waves or 
reflections. The sound field of each projector is then 
computed at several frequencies, from the data ob- 
tained with each standard hydrophone. With random 
deviation from the mean, agreement within 1 db is 
reasonable assurance of projector and hydrophone 
stability. When the differences between the computed 
sound pressures exceed 1 db, it is advisable to include 
the data from a third standard in order to identify the 
cause of the discrepancy. 

Receiving Response Measurements. Instruments 
which have been designed for hydrophone operation 
are calibrated in a sound field established by the pro- 


cedure described above. The instrument is oriented 
acoustically before any test data are recorded, but the 
device may or may not be tuned according to the 
specifications of the program. 

On the basis of an exploratory observation, the 
gain of the receiving amplifier is adjusted so that the 
curve will remain on the chart as the frequency range 
is covered. If the level range of the instrument ex- 
ceeds that of the recorder, it may be necessary to 
change the gain during the course of the run or to 
repeat a portion of the curve at a different gain set- 
ting. Runs at several test distances usually are re- 
corded on the same chart and, if possible, with the 
same gain. 

In the case of sharply resonant devices, supplemen- 
tary records are made of the level at peak frequency 
by slow manual variation of the oscillator. Measure- 
ments at frequencies on each side are made until the 
levels are 3 and 10 db below the peak response. These 
observations permit determining the Q of the in- 
strument. 

In general, the response of essentially nondirec- 
tional apparatus is taken at several angular positions 
such as 0, 90, 180, and 270 degrees. Comparison of 
these runs may show inaccuracies in rigging, particu- 
larly at low frequencies where diffraction effects are 
not likely to occur. 

Hydrophone Coupling Measurements. The re- 
corded data are a measure of the signal level at the 
135-ohm input to the receiving system in db vs 10“^® 
watt. Since the instrument calibration is usually 
wanted in terms of open-circuit voltage, or voltage 
across a specified impedance, it is necessary to deter- 
mine the relationship between these quantities and 
the recorder level. This is obtained by injecting a 
signal from a low impedance in series with the hydro- 
phone circuit, thus simulating the voltage generated 
under acoustic excitation. The difference in level be- 
tween the injected signal and the input to the receiv- 
ing amplifier determines the hydrophone coupling. 
Typical circuit arrangements for observation of coup- 
ling characteristics are shown in circuits A and B on 
Figure 32. 

The hydrophone calibration may be required in 
terms of the voltage delivered to a specific load re- 
sistor. In this case, the hydrophone, bridged by the 
required resistance, is connected to the coupling am- 
plifier. The conversion of the recorded level to the 
voltage at the input of the coupling amplifier re- 
quires a knowledge of the relation of gain to fre- 


EQUIPMENT AT MOUNTAIN LAKES 


95 



B 


Figure 32. Typical circuit arrangements for hydrophone 
coupling characteristic measurements: (A) observation of 
input signal, (B) observation of output signal. 


quency for this amplifier. This relation may be ob- 
tained by using the circuit arrangements shown in 
Figure 33. 

Low-sensitivity, high-impedance hydrophones are 
usually tested in conjunction with the underwater 
preamplifier described in Section 6.2.1. This arrange- 
ment permits the hydrophone calibration to be re- 
ferred to the ends of extremely short leads and results 
in an essentially open-circuit calibration. 

An alternative method of calibration, particularly 
adapted to high-impedance tourmaline gauges, ex- 
presses the hydrophone output in terms of the charge 
generated rather than the open-circuit voltage. This 
method, described in Chapter 4, requires minor modi- 
fications in the underwater preamplifier. 

Measurements of Inherent Noise. The inherent 
noise level of a hydrophone is measured with the in- 
strument in quiet water. When open water conditions 



Figure 33. Circuit arrangements for determination of 
gain frequency characteristic of coupling amplifier: (A) 
observation of input signal, (B) observation of output 
signal. 


are not sufficiently quiet, a bucket with suitable anti- 
shock mounting or an acoustically treated tank is 
used. A highly efficient unit, the noise level of which 
is greatly affected by changes in the radiation impe- 
dance, should not be tested in the bucket because of 
probable standing waves. In fact, even an acoustically 
treated tank may allow the formation of standing 
waves sufficient to prevent exact noise level measure- 
ments. Standing waves cause variations in the im- 
pedance which the medium offers to the diaphragm. 
In well-designed radiators, the mechanical impedance 
of the diaphragm more or less matches the impe- 
dance of the medium to which the energy is trans- 
ferred. The more efficient the device, the less will be 
the loss between the electric power supplied and the 
acoustic power radiated and, therefore, the closer will 
be the coupling between the impedance of the device 
and the impedance of radiation. Since thermal noise 
generally is proportional to the resistance, small con- 
fined areas which produce standing waves are not 
conducive to accurate measurements on the more 
efficient devices. 

In order to minimize mechanical vibration, the test 
units are suspended in low-period antishock mounts 
and every effort is made to reduce the background 
noise to a minimum during the observations. 

Measurements of the hydrophone noise level and 
its distribution through the operating frequency 
range are made on system 2 with each of the accept- 
ance band widths, 10, 300, and 6,000 c, in such a 
manner that adequate overlap is obtained. To deter- 
mine whether the noise level of the system is suffi- 
ciently high to affect the measurement of the hydro- 
phone noise level, observations are made with a re- 
sistor connected in place of the test instrument. The 
resistance is selected by trial to be small enough so 
that the thermal noise generated in it is negligible. 
Progressively smaller values are tried until there is 
no observable change in the output noise level. 

Receiving Directivity Patterns. Directivity patterns 
of transducers may be obtained rapidly and with good 
angular accuracy by means of the rotator and record- 
ing turntable assemblies described in Section 6.2.1. 
Exploratory observations are first made to check the 
angular orientation and to adjust the system gain 
so that the pattern traced will lie within the bound- 
aries of the recorder paper. Whenever possible, the 
signal level at zero angle is adjusted to the upper 
limit of the chart in order to utilize its full 50-db 
range and to facilitate subsequent chart comparisons 


96 


USRL TEST STATIONS 


SYSTEM I 


PIER 1 


TRANSMITTING BOOTH 



PROJECTOR 


Figure 34. Typical circuit arrangement for observations of projector acoustic output versus electric power input. 


by superposition. The rate of rotation is selected on 
the basis of instrument size, driving torque required, 
and pattern complexity. For example, small devices 
may be rotated at the maximum rate, provided the 
rate of signal variations does not exceed the response 
rate of the recorder. 

After the preliminary observations and adjust- 
ments, the pattern is recorded with the turntable 
moving through a complete revolution. Overall sys- 
tem and transducer stability may be checked by not- 
ing the recorded trace at the point of overlap. 

For all critical test conditions, the pattern should 
be repeated at a second distance. If the pattern 
changes radically with distance, this usually indicates 
too small a test distance (see Chapter 5) or the pres- 
ence of reflections. In the former, the distance should 
be increased, while in the latter it should be de- 
creased, the frequency slightly displaced, or other 
means employed to reduce reflections. 

In general, sharply resonant transducers are in- 


vestigated for pattern characteristics at the frequency 
of resonance, at frequencies slightly above and below 
this point, and occasionally at the frequencies of sec- 
ondary and tertiary resonance. 

Design symmetries or asymmetries which control 
the beam pattern are investigated by making records 
about several axes of rotation. The positioning of a 
transducer for an investigation of design symmetry is 
illustrated in the sample circuit sheet shown in Fig- 
ure 3 1 . 

Split transducers, designed for bearing deviation 
indicator [BDI] operation, may be studied for phase 
shift and symmetry by means of a special circuit de- 
signed by the Harvard Underwater Sound Labora- 
tory.'"*^ This circuit provides for the halves of the 
transducer to be in parallel but the connections to 
one may be reversed so that its output may be in the 
same phase or opposite to the other. Lag lines giving 
various phase shifts are readily available and when 
plugged into the circuit are connected to a three- 


EQUIPMENT AT MOUNTAIN LAKES 


97 


position switch which determines that the outputs 
shall match, or chooses the one that shall be made 
to lag. 

The patterns usually taken for the BDI characteri- 
zation of a transducer are for halves in phase with no 
shift, halves in opposite phase with no shift, halves in 
phase with one shifted to lag, and halves in phase 
with the other shifted to lag. The gain chosen for the 
first condition should be maintained for the others. 
The first two patterns are usually recorded on one 
chart and the second two on another. 

Pattern investigations of hydrophones usually are 
made with a continuous sinusoidal frequency, but 
may be made by pulsing within the limits discussed in 
Chapter 5. It is of interest occasionally to take pat- 
terns representing listening conditions with a broad 
noise band covering approximately the audio range. 
A 6-kc band centered at 4 kc is most satisfactory. 

Transmitting Response Measiire?ne? 2 ts. In obtain- 
ing data on transmitters it is not necessary to establish 
a definite sound field. The calibration is made with 
standard hydrophones, selected on the basis of fre- 
quency coverage and uniformity of response at speci- 
fied frequencies. IVIost of the measurements are made 
with only one standard, but this is checked by at least 
one other. This is not only to check the standard hy- 
drophone but also to test the projector stability. Most 
of the preliminary observations are made at low 
power levels and then extended to maximum values. 
Projectors are usually calibrated with continuous 
sinusoidal signals, but tests may be made with pulsed 
ones, as described above. The impedance from which 
the projector is driven is selected to agree with the 
recommended value. A convenient available power 
level is 160 db vs 10“^^^ watt, but it must never exceed 
the recommended maximum. (See Figure 29.) 

Projectors that require an external tuning circuit 
for power factor improvement are calibrated both 
with and without this circuit. Some projectors re- 
quire a polarizing current, usually supplied through 
a universal-type junction box. This box provides for 
the adjustment and metering of the current, and also 
incorporates adjustable capacitance networks made 
flexible by a plug-in design. 

Preliminary observations include checks on leak- 
age, cross talk, noise, overloading, angular orienta- 
tion, and required circuit gain. Following these, re- 
sponse data are taken, usually at two distances, and 
coupling measurements are made for the standard 
hydrophone used. 


L TEST STATION 

PROJ. NO — QO — DATE ?- ^ ‘■^<COPlES TO STATION FILE AND N. Y. OFFICE 

SHEET NO. B V* 

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Figure 35. Typical data sheet, showing projector output 
versus electric power input. 


Transmitting Directivity Patterns. Directivity pat- 
terns about one or more axes of rotation are required 
in a projector calibration to determine the radiation 
field. The procedure for taking these with the polar 
recorder has already been given under Receiving 
Directivity Patterris. 

Load Characteristics. Load-run observations to de- 
termine the relationships between the various projec- 
tor characteristics and the impressed power may be 
made by means of the circuit arrangement shown in 
Figure 34. These data should include observations of 
linearity, frequency shift, distortion, and impedance. 
Additional patterns may be taken to observe the 
variation of any quantity as a function of power. 

Figure 35 is a copy of an original data sheet show- 
ing typical load-run observations. The time entries 
are indicative of the soaking periods allowed for ade- 
quate thermal stability to be reached following each 
change in impressed power. A description of the watt- 
meters with their circuits and theory is given later in 
this chapter in Section 6.2.8. 

Impedance Data. Impedance data are taken on all 


98 


USRL TEST STATIONS 


transducers not directly associated with preamplifiers. 
The devices are usually suspended from the pier, well 
below the lake surface, while the measurements are 
made. Occasionally impedance measurements are 
made with the transducer suspended in the acousti- 
cally treated tank. Such data are checked for standing 
wave effects by repeating a few observations from a 
different position. The temperature of the water ad- 
jacent to the transducers is always recorded and an 
adequate soaking period is provided to permit com- 
plete thermal equilibrium. 

Observations on wide-band transducers that have 
no sharp resonances are taken to cover the frequency 
range in increments which will permit the construc- 
tion of accurate resistance and reactance curves. 
Additional observations are taken on resonant trans- 
ducers in the resonance regions, so that motional 
impedance computations may be made. These ob- 
servations include not only measurements at the 
frequency of maximum response but also at frequen- 
cies above and below this value until the response 
is less by 0.5, 1, 2, 3, 5, and 10 db. 

After considering the bridges available for direct 
impedance measurements, the usual procedure is to 
take the series impedance, if its value is not over 
10,000 ohms and one terminal can be grounded, and 
the shunt admittance for larger values and devices 
that cannot be grounded. Some attention must be 
given to the choice of the frequency that should be 
used. A description of the bridges is given in Section 
6 . 2 . 8 . 

Whenever possible, measurements are made at the 
apparatus terminals but when additional cable must 
be used, measurements are made of the device with 
cable, and of the cable alone, so that a correction for 
the latter may be applied. To facilitate making these 
corrections, all bridge readings are expressed in the 
same unit, that is, impedance or admittance. When 
auxiliary apparatus is used, impedance measure- 
ments are also made on it to allow the computation 
of the values for the instrument itself regardless of 
the circuit used. 

The impedance of a transducer may also be ob- 
tained from the readings of the wattmeter and though 
the accuracy is not as good as with the bridge, it af- 
fords a continuous indication and permits the detec- 
tion of all resonances. It has the further advantage 
that impedance data is obtained at all power levels, 
instead of at the low levels bridges require, and 


impedance as a function of power may be observed. 

Reciprocity Test Procedure 

Several times during the year a series of free field 
reciprocity calibrations are made on all of the labora- 
tory standard hydrophones. Comprehensive periodic 
checks are essential, since most of the calibration 
work is based on an accurate knowledge of the per- 
formance of these instruments. A large part of the 
preparation procedure is identical with that in com- 
parison testing. The instruments are rigged, washed, 
and oriented in the same manner. The required 
measurements include response, hydrophone coup- 
ling, and projector current, but the number and 
sequence differ materially. A discussion of the the- 
oretical considerations involved in reciprocity cali- 
brations and of the general procedure used is 
presented in Chapters 5 and 7. Actual procedures are 
discussed herewith. 

Requisites. A reciprocity calibration requires the 
use of a reversible transducer which obeys reciprocity. 
Since two semi-independent calibrations can be ob- 
tained with little extra effort by using a pair of such 
transducers, this procedure is considered. It may be 
necessary to use several pairs of transducers to cover 
the frequency range from 15 c to 150 kc. In each test 
run, one of the given pair operates as a hydrophone, 
while the other furnishes the sound field. In a re- 
ciprocity calibration it is necessary to know the open- 
circuit voltage of the instrument, although it may be 
more convenient to measure the voltage across a 
known impedance and calculate the open-circuit 
value. The driving and receiving impedances are 
selected to provide the most uniform frequency re- 
sponse. The equations given in Chapter 5 require a 
knowledge of the distances between the acoustical 
centers. The selection of a center is arbitrary but, 
once selected, must remain the same throughout the 
test. It is usually chosen to approximate the center 
of the spherical waves which the transducer produces 
at large distances and for this reason, runs are taken 
at several hydrophone-projector separations. From 
the distance-loss relationships, the effective acoustic 
center of the transducer can be determined. The runs 
also assist in evaluating the effect of reflections and 
standing waves. 

Testing Procedure. It is indicated in Chapter 5 that 
there is no simple a priori test that determines wheth- 
er or not a particular transducer obeys the reciprocity 


EQUIPMENT AT MOUNTAIN LAKES 


99 


principle. The most satisfactory substitute is a cross 
check between two reversible instruments separated 
by a fixed distance. AVhth one functioning as a hy- 
drophone and the other as a projector, the ratio is 
obtained of the open-circuit voltage developed by 
the one to the current supplied the other. The elec- 
tric connections are then interchanged and the runs 
repeated, giving the same ratio with the function of 
each reversed. If the two transducers obey reciprocity, 
these ratios will be equal at each frequency. It is pos- 
sible that these ratios may be equal, even though the 
transducers do not obey reciprocity, but there is little 
chance of this unless the transducers are identical, in 
which case they may violate the reciprocity principle 
to the same degree. 

When reciprocity has been established, the trans- 
ducers are placed at opposite ends of the testing area 
with no change in their orientation or depth. The 
hydrophone to be calibrated is then mounted on a 
movable carriage between the two and tested against 
each in turn. The procedure outlined in Chapter 5 
is then carried through, using each transducer as a 
projector and as a reversible transducer as described. 
This procedure yields two semi-independent recip- 
rocity calibrations of the hydrophone. It is usual to 
test successively all hydrophones of the same type, 
since this procedure entails a minimum number of 
changes in rigging and electric connections. 

Other Free Field Acoustical Observations 

In addition to the usual calibrations, acoustic stud- 
ies are made which involve somewhat different test- 
ing techniques, such as observations on domes, baffles, 
generators of complex acoustic waves, and complete 
echo-ranging systems. 

Dome Studies. When a dome is submitted for in- 
vestigation, the projector to be used with it may not 
be included and a suitable one must be selected. It 
may be a wide-band device or a sharply resonant 
one, but it must have low side lobes at the frequencies 
of interest and have as nearly as possible the same 
size and directivity as the projector for which the 
dome was designed. This is very important, since a 
projector with unsuitable directional characteristics 
can completely obscure important dome charac- 
teristics. 

The transducer is usually tested first without the 
dome by rigging to the inner shaft of the rotator. The 
measurements ordinarily include frequency response 


as a hydrophone or as a projector and directivity 
patterns as a hydrophone. The character of the trans- 
ducer will largely determine the frequencies at which 
the dome is tested. If the transducer is sharply reso- 
nant, patterns may be taken only at the resonant fre- 
quencies. 

Following these tests the dome is thoroughly 
cleaned and rigged to the outer shaft of the rotator 
with care being taken to insure correct positioning of 
dome to transducer. The assembly is then debubbled 
and allowed to reach temperature equilibrium. 

The type and character of the test data required 
for a comprehensive investigation are illustrated in 
Table 1. Figure 36 shows the various configurations 
of the acoustic gear. Because of reciprocity, as shown 
in Chapter 5, the effect of the dome, the baffle, and 
their surroundings on the response and directivity 
of the transducer is the same on sending and receiv- 
ing. For practical reasons, receiving is preferred. 

There are considerations in making dome directiv- 
ity tests which require somewhat different arrange- 
ments. To investigate the effects of either a given 
target or a noise source located at a definite bearing, 
the nose of the dome is set at the bearing and the 
transducer alone is rotated as indicated in Figure 
36D. The same procedure with the dome reversed, as 
shown in C, is used to simulate the propeller noise 
from one’s own ship. The effect of the dome on the 
pickup of water noise and reverberation for any par- 
ticularly trained position ^ of the transducer can 
best be shown by rotating the dome and projector 
together with the angle between them fixed during 
each trial (Figure 36E). This plan calibrates the re- 
sponse of this combination of dome and transducer 
to noise from any direction. 

Accurate positioning of the dome and hydrophone 
is easily accomplished by means of the rotator and 
turntable. Either rotator shaft may be locked in posi- 
tion while the other is rotated. It is thus possible to 
secure the required angular separation between the 
hydrophone face and the dome nose. By engaging 
both shafts, the whole assembly may be rotated while 
the relative angular position of dome and hydro- 
phone remains fixed. 

Baffle Studies. The reflective and absorptive prop- 
erties of baffles may be studied independent of domes. 
While no direct measure of absorption is made, this 
characteristic may be estimated from reflection and 
transmission measurements. The testing arrange- 


100 


USRL TEST STATIONS 






V. 


\ 


c 



0 

ANGLE NOTATION 

O = ANGLE OF ACOUSTIC AXIS OF TRANSDUCER 
WITH RESPECT TO DIRECTION OF SOUND 
PROPAGATION 

6 = ANGLE OF DOME WITH RESPECT TO DIRECTION 
OF SOUND PROPAGATION 

<l) = ANGLE OF DOME WITH RESPECT TO ACOUSTIC 
AXIS OF TRANSDUCER 

/= CONTINUOUS ROTATION THROUGH INDICATED 
ANGULAR RANGE 


Figure 36. Typical testing geometries for dome and baffle measurements: (A) reference inns, (B) transmission through 
nose of dome, (C) transmission through baffle, (D) dome loss versus angle, (E) specular reflection studies. 


ments are shown in the sketches of Figure 37. Com- 
parison of the reference pressure obtained between 
the hydrophone and projector with and without the 
baffle gives the insertion loss characteristic of this 
baffle. A sound pressure is present on the far side of 
the baffle because of transmission through the baffle 
and diffraction around it. This is more fully discussed 
in Chapter 9. 

I'he reflecting and absorbing properties of the 
baffle may be further studied by modifying the testing 
setup as shown in Figure 37D. The baffle and trans- 
ducers are positioned so that the over-all path length 
of the reflected sound is equal to d. With the baffle 
removed, a screen is placed between the projector and 
hydrophone as in Figure 37C to minimize the direct 
transmission. The magnitude of the sound received 
determines the threshold in the reflection measure- 
ments. If the sound pressure measured by the hydro- 
phone, with baffle in place as in Figure 37D, is ecjual 
to that measured in the reference run of 37A, the 


baffle is a perfect reflector. The amount by which the 
two differ is a measure of the sound transmitted, ab- 
sorbed, and diffracted by the baffle. 

Studies of Acoustic Properties of Various Materials. 
Occasionally the acoustic properties of a particular 
material are investigated to determine its suitability 
for a proposed application. Reflection and transmis- 
sion tests, similar to those made on baffles, are made 
on samples of the material. These should be large 
enough to avoid diffraction in the frequency range 
under consideration. Before testing, the samples are 
thoroughly washed, debubbled, and submerged until 
they are in temperature equilibrium with the water. 
Often additional tests are made to discover the criti- 
cal angles at which incident sound will be completely 
reflected by the material. The sample is rotated in the 
setup of Figure 37B and the angles observed at which 
minimum transmission occurs. This information is 
particularly valuable in the design of dome windows 
and in the selection of materials for them. 


EQUIPMENT AT MOUNTAIN LAKES 


101 


Tablk 1. Test data required tor dome investigation. 


Purpose of 
observation 

Angidar 

position* 

Arrange- 
ment in 

a 

d 

= d — a 

Fig. 36 

I'ransducer response 
without dome 

0° 



(A) 

Transducer pattern 
without dome 

|360° 



(A) 

Transmission response 
through nose of dome 

0° 

0° 


m 

Transmission response 
through rear of dome 

0° 

00 

o 

o 


(C) 

Transducer pattern 
with baffle and rear 
of dome interposed t 

|360° 

00 

o 

o 


(C) 

Transmission pattern 
through dome vs 
angidar position 

0° 

^360° 


(D) 

Transducer patterns . 
through dome 

|360o 

|360° 

0° 

15° 

30° 

45° 

etc., inch 
known critical 
angles 

(E) 


* Transducer used as a receiver only. 

t If the dome is equipped with a removable baffle, tests 
should be made with (a) dome and baffle, (b) dome alone, and 
(c) baffle alone, to see how baffle and dome affect noise pickup 
over a range of angles about the rear of the transducer. 


Studies of Acoustic Waiw Signal Generators of 
Complex Waves. Devices designed to generate acous- 
tic signals of complex wave forms are divided into 
two major categories, each requiring a special testing 
technique. In general, the choice of technique is 
based on such factors as a continuous signal, recur- 
rent signal and its rate, and the crest factor (ratio of 
peak to rms values). 

Obviously, the use of electromechanical ecpiip- 
ment for the recording of random events is limited by 
such factors as the speed and dynamic range of re- 
corder response. Signals that are intermittent, that 
have low recurrence rates, and those with crest factors 
greater than 2.5, are usually studied by means of a 
cathode-ray oscilloscope and high-speed oscillograms. 
Signals that are continuous or of high recurrence rate, 
with crest factors not exceeding 2.5, may be studied 
with the apparatus of system 2. The procedure for in- 
vestigating such signals with system 2 is stated here- 
with and may be used for those of periodic or aperi- 
odic nature. 

The devices are rigged and positioned with the 
usual procedure for testing projectors. Provision is 



BAFFLE 

PROJECTOR n HYDROPHONE 



PROJECTOR 




FuaiRE 37. Plan view of typical test set-iqDS for studying 
baffle characteristics: (A) reference runs, (B) sound trans- 
mission through baffle, (C) threshold signal without 
baffles, (D) sound reflection by baffle. 


made for the observation and control of the driving 
power. A standard hydrophone having a substantially 
uniform frequency response is selected for the acous- 
tic pressure measurement. In the selection, the maxi- 
mum instantaneous pressure must be considered in 
order to avoid overloading of the preamplifier. 

Observation of the crest factor should be made first 
with the arrangement shown in Figure 38, where the 
rms of the signal is obtained on the 30A set and the 
peak amplitude on the cathode-ray oscilloscope. The 
complex wave signal is then replaced by a sinusoidal 
one from an oscillator set at a level which gives the 


102 


USRL TEST STATIONS 



Figure 38. Simplified circuit arrangement for determina 
tion of peak factors. 


same deflection on the oscilloscope as the test signal. 
The difference between the 30A set readings on the 
test and oscillator signals gives the difference in db 
between the crest factor of the complex wave and that 
of the sine wave. Since values are being determined in 
volts and a sine wave has a crest factor of 1 .4 1 (= 3 db), 
this amount has to be added to the observed differ- 
ence in db before reverting to the ratio that expresses 
the crest factor of the complex wave. 

The investigation includes observations of the total 
(broad-band) energy and of the energy distribution 
with respect to frequency from 100 c to 150 kc with 
the receiving amplifier output connected directly to 
the broad-band recorder. The examination of energy 
distribution with respect to frequency over the test 
signal spectrum can be made with the normal facili- 
ties of system 2, which has three acceptance bands in 
the detector circuit, 10, 300, and 6,000 c. As these 
bands become a continuously smaller percentage of 


the frequency as it increases, the 10-cycle band usually 
is carried only to 20 c and the 6-kc band is not used 
below 4 kc. When the recurrence rate of the signal is 
too low, the recorder will follow the individual cycles 
which involve the difficulty of evaluating wide and 
irregular traces. Suitable resistors are inserted in the 
pen-drive motor circuit to reduce the response rate 
and thereby minimize the recorder excursions. 

The energy in band widths other than those pro- 
vided by the detector circuit may be obtained by the 
use of the circuit arrangement shown in Figure 39. 
The plan of this circuit is to transpose the signal fre- 
quencies by heterodyning them so that the desired 
signal band may be obtained with available filters. 
The method is illustrated in Table 2, which gives the 
value of the signal frequency at various stages, with 
the circuit adjusted to pass only a band of frequencies 
1 kc wide centered at 10 kc. 

Band widths commonly used in such investigations 
include 5 ± 0.25 kc, 10 ± 0.5 kc, 20 ± 1 kc, 30 ± 1.5 kc, 
and 40 ± 2 kc. It is obvious, however, that the system 
may be adjusted for any band width and mid-band 
frequency requirement within the limits indicated on 
Figure 39. 

A determination of the rate of signal recurrence is 
usually made by inspection of the broad-band re- 
corder traces. For high values, the speed of paper 
drive on the recorder should be at a maximum. 

The investigation of the operational stability or 
life characteristics of expendable devices is made by 


Table 2. Value of signal frequency in kilocycles per second at various stages 
when circuit is adjusted to pass all frequencies in the band 10 ± 0.5 kc. 


The dial setting of the heterodyne oscillator is 12.5 kc to obtain 637.5 kc. This is the sum of the lowest signal frequency to be 
passed and the half-band width of the filter in the detector circuit added to a fixed frequency. 

Frequency of oscillator No. 3 is set at the sum of upper cutoff frequency of the detector filter and the cutoff frequency of 
the low-pass filter minus the desired band width. 

XXX indicates the filter that cuts off the frequencies outside the desired band. 


Detector circuit 
94-100 kc 


15 kc 


Freq. of 
signal 

Var. freq. 
osc. No. 2 

Mod. No. 1 
output 

Mod. No. 2 
output 

B.P. filter 
output 

Osc. No. 3 

Mod. No. 3 
output 

L.P. filter 
output 



A + R 

747-C 



F- E 


9.0 

637.5 

646.5 

100.5 

XXX 




9.5 

” 

647.0 

100.0 

100.0 

114.0 

14.0 

14.0 

10.0 

” 

647.5 

99.5 

99.5 

” 

14.5 

15.5 

10.5 

” 

648.0 

99.0 

99.0 

” 

15.0 

15.0 

11.0 

” 

648.5 

98.5 

98.5 

” 

15.5 

XXX 



103 


EQUIPMENT AT MOUNTAIN LAKES 



Figure 39. Simplified circuit arrangement using System 2 with auxiliary apparatus for obtaining a band-pass filter 
adjustable in width up to 6 kc and with a mid-band frequency variable between 3 and 150 kc. 


permitting the recorder to run for an adequate pe- 
riod with the circuit arranged for broad-band ob- 
servations. The synchronous motor of the paper drive 
measures the time of these observations and the paper 
speed should be at a minimum. 

The circuit arrangement in Figure 40 permits ob- 
servations on the effectiveness of complex acoustic 
signals in aural masking. A variety of characteristic 
sounds on phonograph records is available with 
which to study masking effects at sonic or supersonic 
frequencies. 

Operational Studies of Complete Echo-Ranging 
Systems. With the apparatus connected into a com- 
plete system for echo ranging, actual trials are made 
to test the ability for locating small objects in shallow 
water at distances under 350 meters. A common meth- 
od is to suspend a small target from a rowboat which 
travels slowly over the range while the attempt is 

SYSTEM 2 



made to locate the target and maintain contact with 
it. Common targets are 2- and 3-foot hollow spheres 
and a 4-foot length of 7-inch heavy walled pipe. One 
of these is suspended from the stern of the boat at the 
same depth as that of the transducer. Directions for 
the course of the boat and other communications be- 
tween operators are carried on by radio telephone. 

The raft has a working space running its full 
length through the center. The apparatus rests on 
carriages moving on steel rails the same as on the 
piers while an overhead rail and hoist facilitates posi- 
tioning on the carriages. The monorail on pier 1 
offers the easiest transfer of equipment to the raft, 
which must be kept level by proper weight distribu- 
tion. With the gear in place, the raft is located at one 
end of the range and secured to the shore by 1-inch 
cables with wide angular spacing. Connections are 
made to the power and signal lines, which include a 


PHONOGRAPH 

SYSTEM 



Figure 40. Simplified circuit arrangement for acoustic masking observations. 



104 


USRL TEST STATIONS 


telephone to the laboratory. The raft carries perman- 
ently a 6-kva, 60-cycle regulator that will maintain 
any voltage from 105 to 125. Canvas drops are sup- 
plied to protect the operators in bad weather. 

Temperature gradients in the lake are checked 
by bathythermograph records taken several times 
throughout the test period. Since factors such as gas- 
bubble accumulations on the bottom may affect the 
acoustic conditions, it is advisable to make simultane- 
ous observations on a reference system maintained 
at the laboratory for this purpose. 

6.2.3 High-Frequency System 

General Description 

The high-frequency system at the Mountain Lakes 
laboratory covers the range from 100 kc to 2.2 me. 
While the electronic system is capable of calibrating 
up to 3.5 me, no standard transducers are available 
for those frequencies. 

The system comprises the following items: 

1. An electric system capable of producing, meas- 
uring, detecting, and recording signals in the fre- 
quency range 50 to 3,500 kc. 

2. An indoor calibration tank with the necessary 
mechanical equipment for positioning and aligning 
the units under test. Included here also are the vari- 
ous acoustical devices for the reduction of reverbera- 
tion. 

3. Two sets of mechanically interchangeable trans- 
ducers having overlapping frequency responses, with 
one set covering the range 100 to 800 kc and the other 
300 to 2,200 kc. 

4. Mechanical and electrical components for meas- 
urements in the outdoor test areas. These subjects 
will be treated in detail in the following paragraphs. 

Electric System 

The electric system consists of a heterodyne oscil- 
lator, power amplifier, and coupling network for 
driving a projector, and of a detector and recording 
circuit coupled through a preamplifier for measuring 
the output of a hydrophone. This system will make 
continuous ink recordings of the combined response 
of a pair of instruments throughout the frequency 
range 50 to 3,500 kc. A block diagram of this system 
is shown in Figure 41. The frequencies and signal 
transmission direction are also shown on the diagram. 

Oscillator. The heterodyne oscillator consists of 
one oscillator fixed at 15 me and the other variable 


from 1 1.5 to 15 me. Each feeds through its own buffer 
to the same modulator, which is followed by a filter 
designed to pass only the difference frequencies of the 
oscillators. An amplifier stage follows the filter and 
terminates in a transformer designed to supply a 
72-ohm impedance. A standard attenuator of this 
value is inserted between the transformer and the 
output jack of the oscillator. 

As the block diagram indicates, a small portion of 
the output is rectified, converted, amplified, and fed 
into the buffer stage for the fixed oscillator. This 
constitutes an automatic volume control [AVC] 
which will hold the output within 0.15 db over the 
entire frequency range, while without it, the varia- 
tions may be 1.5 db. The maximum output of the 
oscillator is 222 milliwatts corresponding to 4 volts 
across 72 ohms and the harmonic level is 45 db below 
that of the fundamental frequency. 

The mechanical construction is very similar to that 
of the other systems. The variable-frequency oscil- 
lator can be driven either by hand or by a synchron- 
ous motor which allows the entire range to be cov- 
ered in 1.5, 4.5, or 13.5 minutes. The calibrated scale 
is a 30-foot strip of 35-millimeter film. Values may 
be determined between the lines of this scale by in- 
terpolation with a vernier dial. 

The oscillator frequency is adjusted to the scale 
calibration at 76 kc by means of a high-Q tuned cir- 
cuit and at 2 me by means of a quartz crystal. These 
adjustments are made by a trimmer condenser and 
a small adjustable inductance in the circuit of the 
variable oscillator. A frequency check is made several 
times each day to correct for any drift. This effect, 
however, becomes negligible after 72 hours of con- 
tinuous operation and accordingly, the power is not 
turned off, unless it is to be for a period of several 
days. 

Power Amplifier and Power Level Measurements. 
The power amplifier is the wide-band type, covering 
50 to 3,500 kc with a gain of 35 db flat to 0.5 db. The 
input and output impedances are both 72 ohms and 
the maximum power available is 25 watts (174 db vs 
10-16 watt). It should be noted that the output of this 
amplifier can be either balanced or unbalanced with 
respect to ground. 

As indicated on the block diagram, a power level 
measuring set is used to measure the available power 
at the output of the oscillator or power amplifier. 
This instrument is a high-frequency equivalent of 
the Western Electric 30A transmission measuring set. 


EQUIPMENT AT MOUNTAIN LAKES 


105 



Figure 41. lilock diagram of high-frequency system. 


It consists essentially of a high-frequency thermocoii- 
j)le calibrated on direct current and matched to 72 
ohms. When properly used, this instrument will meas- 
ure the power dissipated in a 72-ohm load. 

Projector Coupling Netiuork. The purpose of the 
projector coupling network is to match the imped- 
ance of a projector to that of the driver and the 
associated transmission line. Arrangements are also 
provided for measuring the current to the projector. 

The most common network is a transformer that is 
designed for standard instruments over this range of 
frequencies. The majority of the projectors encount- 
ered have the same general characteristics as the 
standards and so operate satisfactorily with this trans- 
former. If a closer match is required, various resist- 
ances may be added to bring the impedance to the 
desired value. 

The transformer for high-frequency standards has 
a nominal impedance ratio of 72:1,000 ohms and is 
designed especially for a capacitive (crystal) load. 
Other ratios used are 72:20, 72:72, 72:250, and 
72:2,000. 


Hydrophone Coupling Circuit. For high-imped- 
ance hydrophones the usual coupling circuit is a 
high-impedance input amplifier. Since it is necessary 
to match the coupling circuit to a low (72-ohm) im- 
pedance transmission line, it is convenient to look 
upon these coupling circuits as impedance trans- 
formers. One of the best electronic circuits for this 
purpose (an electron tube is necessary because of the 
high input impedance requirements) is a cathode- 
follower type circuit. These circuits used in connec- 
tion with underwater acoustic devices are known as 
preamplifiers. The single-stage preamplifiers used in 
the high-frequency system have an input impedance 
of about 25 megohms in parallel with about 5 /x/xf. 
The output impedance is 72 ohms balanced or un- 
balanced and the gain is —20 db flat to within 0.25 
db over the entire range. One preamplifier is de- 
signed particularly for the high-frequency standards, 
while the others are portable for use in current meas- 
urements and with other hydrophones. 

It should be noted that the abnormally high values 
of grid resistors are not necessary as at the lower fre- 


106 


USRL TEST STATIONS 


quencies because the input capacity is the controlling 
impedance. 

A calibrating resistor has been included in these 
preamplihers to obtain the coupling loss of the cir- 
cuit, which is required in order to evaluate the open- 
circuit voltage of the hydrophone. 

Wide-Band Amplifier. The wide-band amplifier 
shown on the diagram has a gain of 40 db and is flat 
within 0.25 db from 50 kc to 3.2 me. At 3.5 me the 
response is down 3 db. Both the input and output im- 
pedances are 72 ohms. 

The purpose of this amplifier is to increase the 
signals that are too small to record, even with the 
full gain of the detector. 

Since the amplifier is unbalanced, it is associated 
with a combination of 72: 72-ohm coils which allows 
changing the condition to ground from unbalanced 
to balanced and vice versa. By means of terminations 
on the coaxial jack-strips, the coils and the amplifier 
may be connected to any portion of the circuit, 
though they are normally used as shown in the dia- 
gram. 

Detector. The action of the detector is best ex- 
plained by tracing through a signal from the input. 
The signal frequency is first amplified and then im- 
pressed upon a modulator which receives another 
signal from the variable-frequency oscillator, the 
value of which is 15 me — /. 

One of the modulation products (15 me — / + / = 1 5 
me) is selected, filtered, amplified, and led to a second 
modulator. The second signal to this modulator is 
from a local oscillator with a frequency of 15.097 me, 
which gives 97 kc as one of the modulation products 
of this stage. This signal is carried through a band- 
pass filter centered at 97 kc and 6 kc wide to succeed- 
ing stages of amplification. 

The reason for bringing the 15 me — / signal to the 
detector is that it ties the detector so completely to the 
oscillator that it will detect no signal more than 3 kc 
on either side of the one to which the oscillator is 
set. The principal advantage of this plan is that since 
most of the amplification takes place in a channel 6 
kc wide, a much better signal-to-noise ratio is ob- 
tained. One other advantage is relative ease of record- 
ing at a single frequency (97 kc) as compared with 
wide-band recording. The final stages consist of am- 
plifiers and buffers for the 97-kc signal. A meter with 
a logarithmic scale may be connected beyond these 
stages to indicate the signal level. This is particularly 


useful when aligning and positioning the instru- 
ments in the tank. 

The purpose of the buffer stages is to isolate vari- 
ous circuits from each other. One buffer output goes 
directly to the rms detector and recorder. The others 
are used for monitoring and for making frequency 
adjustments. 

To correct for drift in the 15.097-mc oscillator, pro- 
vision has been made for checking and maintaining 
the 97-kc carrier in the center of the 6-kc pass band. 
A visual indicator of frequency drift such as that 
used in the other systems is not practical because of 
the relatively larger drifts in this system and because 
the eye is unable to perceive flickers above 30 per sec- 
ond. As mechanical shock alone may change the 
frequency as much as 100 c, tone discrimination by 
the ear is the method used. The 97-kc signal is led to a 
heterodyne listener consisting of a 94- to 100-kc oscil- 
lator, a modulator stage, and a loudspeaker. If the 
listener is set at 97 kc, no tone will be heard from the 
speaker when the carrier is 97 kc, but any drift from 
this will be observed as a beat-frequency tone. A 
separate quartz-crystal oscillator of 100 kc is avail- 
able for calibration of the heterodyne listener. 

As is the case with all balanced modulator circuits, 
spurious signals will arise if there is lack of proper 
balance. As the first modulator stage in the detector 
is of this design, some provision must be made for 
indicating the point of balance. In this particular cir- 
cuit, the modulator unbalance results in an ampli- 
tude modulation of the 97-kc carrier, which is 
greatest when the signal in the detector input is 97 
kc. Using a cathode-ray oscilloscope to observe the 
97-kc carrier envelope, the balance controls are ad- 
justed for minimum amplitude modulation of the 
carrier. Audible monitoring of the carrier can be used 
in conjunction with the oscilloscope and provides an 
extremely sensitive indication of the balance point. 
The tone at this point will be noticed to lose its 
“mushiness. " 

The external meter circuit shown in the block 
diagram is somewhat similar to the meter circuit in 
the detector but has the advantage of being portable, 
which allows its use for aligning the acoustic instru- 
ments in the pier test areas away from the electric 
system. 

The detector input is 72 ohms unbalanced and the 
gain of the detector plus recorder is such that 75 db 
vs 10-1® watt into 72 ohms is required to produce a 


EQUIPMENT AT MOUNTAIN LAKES 


107 


full-scale deflection on the recorder. The noise level 
under these conditions is off the lower end of the 
recorder scale, which means that it is below 30 db vs 
10-1® ^ratt into 72 ohms. 

Recorder. The recorder circuit used in this system 
is practically identical with those of the lower fre- 
quency systems and operates only at 97 kc. 

Frequency-calibrated paper is available but only 
for one relative speed of oscillator to recorder. How- 
ever, if it is necessary to make records at other speeds, 
calibrated transparent scales may be used. 

Indexing circuits for marking uncalibrated paper 
at predetermined points on the oscillator scale are 
available. Indexing, however, is done only when a 
special paper is being used, or when directivity pat- 
terns are being taken. 

Noise Generator. As shown in the block diagram, 
the noise generator may be substituted for the fixed 
oscillator at the entrance to the first buffer stage in 
the high-frequency system. The noise signal is a 
6.6-kc band centered at 15 me and, as this is hetero- 
dyned with the variable-frequency oscillator, the re- 
sulting output is the same width centered at the 
frequency /. 

The random noise signal is first generated by a gas 
discharge tube (RCA 150-30), amplified and fed 
through a 1.1 -kc band-pass filter centered at 455 kc. 
It is then used to modulate a 2.095-mc signal from a 
crystal oscillator, and the products in the band cen- 
tered at 2.5 me are selected, doubled, and then tripled 
in frequency. This gives as an output a band of noise 
6.6 kc wide, centered at 15 me. The maximum signal 
level is approximately 0.5 rms volts across 72 ohms. 

The addition of a narrow band-pass filter and a 
local 455-kc oscillator in the noise generator makes 
it a very useful tool for accurately aligning the fixed- 
frequency oscillator at 15 me. This is done by first 
adjusting the variable oscillator to one of the film 
scale calibration points with the noise generator and 
filter circuit substituted for the fixed oscillator. The 
fixed oscillator is then replaced in the circuit and its 
trimmer capacitors adjusted until the same calibra- 
tion point is attained. 

Power Requirements and Power Supplies. The en- 
tire electric system is powered from the regulated 
115-volt, 60-cycle source available in the laboratory. 
The filament supplies to all the tubes are constant 
voltage transformers which give additional regula- 
tion to the electronic circuits. This type of trans- 



Figure 42. Electrical equipment of high-frequency system. 


former may be used, since the harmonics arising from 
this regulation do not interfere with the measure- 
ments. This becomes obvious from the fact that the 
lowest frequency detectable with this system is 50 kc; 
this simplifies considerably the various power and 
distribution requirements. 

The plate supplies are three units of regulated d-c 
power which can supply up to 450 milliamperes at 
300 volts and are identical with the units used on the 
intermediate-frequency systems. 

For the low-level signal circuits (hydrophone pre- 
amplifiers and wide-band amplifier) a special supply 
was constructed with higher regulation than the 
above. This unit furnishes 150 milliamperes at 140 
volts with a stabilization ratio of approximately 
1:10,000 and an internal resistance of about 1 ohm. 
A similar power supply is available for use with the 
outdoor test equipment and is built into the same 
portable case with the external meter circuit. 

Transmission Line and Jack Fields. The photo- 
graph of the electric system (Figure 42) shows the 
transmitting jack field below the oscillator and the 
receiving jack field below the detector. Transmission 


108 


USRL TEST STATIONS 


li^es connect each. oLthese fields with the correspond- 
ing junction box at each end of the tank. There are 
also crossties between the boxes as well as between 
the jack fields. 

With the exception of the control lines for index- 
ing, reporting, and directing, all lines and crossties 
are standard rubber-covered coaxial cable and termi- 
nations are all coaxial jacks (Western Electric Type 
464A). This type of line and terminal is also used on 
the d-c power leads to the hydrophone preamplifiers, 
since in some instances pickup in the power leads will 
be impressed on the signal leads by stray coupling 
capacities in the preamplifiers. Connections to the 
jack fields and to the instruments are made with 
patch cords terminating in standard coaxial plugs 
(Western Electric Types 337A and D 122403). The 
control lines are all standard twinex cable terminat- 
ing in standard telephone jacks (Western Electric 
Type 218A). The use of such standard jack fields and 
patch cords greatly enhances the flexibility of the 
system. 

Lines from other systems also terminate in the jack 
fields. This allows interchange of equipment between 
systems for special testing. Another feature of the 
arrangement is that the lines to other systems can be 
extended to the pier installations by means of their 
jack fields, thus making measurements in the out- 
door test convenient. 

The characteristic impedance of the lines is about 
72 ohms and the line loss is negligible in most cases. 
For example, the loss on a complete loop from the 
high-frequency room out to the end of the pier and 
back again (about 425 feet) was about 1 db at 2.2 me 
and only 2 db at 3.5 me. However, it must be 
emphasized that, unless the lines are properly ter- 
minated, standing waves will occur and lead to er- 
roneous results. 

Ground Loop and Cross Talk Problems. Since 
ground loop effects and cross talk depend on the 
components of particular apparatus and their ar- 
rangement in the circuit, it is almost impossible to 
specify conditions which will apply to all circuits. 
This is particularly true at the high frequencies and 
the low signal levels encountered in this system. How- 
ever, some initial precautions are mentioned here. 

The first is the use of good shielding which must 
be extensively employed to obtain satisfactory re- 
sults. In all the electronic circuits heavy copper shield 
cans must be used to isolate individual stages in each 
amplifier, oscillator, etc. It has even been found nec- 


essary to use copper shield cans around the metal 
tubes in order to decrease radiation. 

The same precautions have to be used on all ex- 
ternal wiring. In most cases, the coaxial cable used 
for transmission lines has two shields, one wound on 
top of the other, for more adequate results and all 
connections are made by coaxial jacks and plugs. It 
is important to remember that it is just as necessary 
to shield the high-level lines as the low-level ones. It is 
even necessary to shield the power leads, particularly 
those to the low-level circuits. 

Adequate filtering (decoupling) must be used in 
each individual stage for all electronic equipment 
and extended, in most cases, even to filtering the fila- 
ment supplies. The latter is particularly true in the 
case of cathode- follower circuits. In addition, ex- 
treme care must be exercised in the choice of ground 
conditions in each individual stage. All of these pre- 
cautions are necessary because of lead inductance and 
stray capacities. In fact, at these frequencies some 
commercial capacitors will appear inductive and 
some commercial inductances, capacitative. Resis- 
tors must be especially designed to operate at these 
frequencies. 

Even with all the above precautions, trouble due 
to ground loops may still arise. This problem is over- 
come by a variety of precautions. The first and most 
important is to determine a good ground and connect 
all other grounds to it. In this case, the tank is se- 
lected and the heavy copper straps in the bays of 
electronic equipment are connected to it by No. 2 
copper wire. The tank is connected to the fundamen- 
tal ground at Pier 2 by No. 0 copper wire. Once the 
grounds are established, it is necessary to connect all 
shielding to them in such a manner as to minimize 
ground loops. 

Since the ground loop conditions change as the 
experimental setups change, it is necessary to con- 
struct the system with the greatest flexibility in 
grounding. This is done by isolating from ground 
the shields of transmission lines and similar equip- 
ment and carrying the connections from the shields 
through the patch cords to the particular piece of 
equipment terminating the line. The shields could 
be connected to ground at this point or isolated by 
means of a doubly shielded transformer. In general, 
it was found best to ground the line at one end only, 
and leave the other end floating. However, all these 
matters are subject to experimental conditions. 

The subject of ground loops and cross talk would 


EQUIPMENT AT MOUNTAIN LAKES 


109 



Figure 43. \'iew of liigh-frecjuency caliiiration tank, shotving ineclianical equipment for holding and positioning the 
instruments. 


not be complete witliout emphasis on internally 
doubly shielded transformers, d'hese make it possible 
to transmit or to receive, at will, on balanced or on 
unbalanced lines. Under most circumstances, bal- 
anced lines are much more effective in reducing cross 
talk and the use of the internal shields on the trans- 
formers results in the lumping of all stray capacities 
between the shields, reducing cross talk still further. 
1 hese transformers also allow a better treatment of 
the ground loop problem and are used for coupling 
between all circuits and lines. 

Acoustical Sy.stem 

I'he acoustical system, in general, consists of an 
indoor calibration tank, mechanical equipment for 
holding and positioning the instruments, absorbers 
for reverberation control, transducers, and outdoor 
test e(|uipment. 

Calibrations may be made in the tank from 80 to 
2,200 kc. rite lower limit is determined by the rever- 


beration, while the upper limit is imposed by the 
response of the standard transducers. 

Calibration Tank of Positioning Eq'nilnnent. The 
calibration tank is made of %-inch steel and has an 
elliptical cross section with approximate dimensions 
of 7 feet for the major axis and 4 feet for the minor. 
The depth of the tank is 4 feet. 

The tank has a capacity of 650 gallons and is 
equipped with a drain and a water inlet at the bot- 
tom and an overflow pipe at the top. Water is pumped 
directly from the lake, and as a result the problem of 
fungus and slime assumes major proportions. Water 
from a community system, as a rule, will contain the 
same organisms, but in much smaller numbers. So 
many different materials are used in the tank that 
care must be exercised in the choice of a fungicide. 
It must be noncorrosive, nonpoisonous, odorless, and 
only slightly electrolytic. Furthermore, it must have 
no effect on rubber. Experience showed that a 2 to 5 
])er cent solution of sodium dichromate was excellent 





110 


USRL TEST STATIONS 


except for its staining and slightly poisonous nature. 
However, a commercial product, “Nalco 21M, ’’manu- 
factured by The National Aluminate Corporation, 
was finally obtained and used quite successfully. This 
substance comes in small briquettes which are placed 
in a holder in the inlet water line and requires no at- 
tention except replenishing. Its only disadvantage is 
that it makes the water slightly milky, as do most 
products of this nature. Its poisonous qualities are 
negligible as long as the concentration is kept at 3 
to 5 parts per million. 

The positioning equipment of the transducer (Fig- 
ure 43) consists of two heavy carriages which can be 
moved the length of the tank on two steel rails, 
mounted lengthwise on top. Each carriage provides 
for positioning the transducer by one screw parallel 
to the long dimension of the tank and a second at 
right angles. The vertical adjustment is made by four 
screws driven at the same rate by a sprocket chain. 
Rotation may be made about a vertical axis for a 
full circle. 

In addition to these degrees of freedom, one car- 
riage is provided with two more adjustments of the 
transducer suspension. One permits the vertical angle 
of the transducer to be changed and the other allows 
it to be displaced with respect to the axis of rotation. 
These arrangements are necessary to permit adjust- 
ment of the direction of the acoustic axis of the trans- 
ducer in the vertical plane lengthwise of the tank and 
to permit rotation of the transducer about an axis 
through its acoustic center. This same carriage has 
motor-driven rotation about the vertical axis to fa- 
cilitate the taking of directivity patterns. 

The transducer holders are designed to include 
the cylindrical case of the preamplifier. Adapters are 
provided for suspending the nonstandard transduc- 
ers. The positioning equipment is very rugged in con- 
struction in order to reduce vibration and other 
extraneous motion to a minimum. This is necessary 
because of the small wave lengths and extremely 
sharp directivity patterns involved. In addition to 
the two carriages on the guide rails, platforms are 
available for suspending other equipment for more 
complex measurements. 

For tests in the outdoor areas, two extra T rails are 
available that can be suspended from the guide rails. 
When these rails are fastened in position, the instru- 
ment platforms can be mounted on them in the same 
manner as on the top of the tank. Simpler rigging, 
consisting of a plate with several cylinders attached. 



Figure 44. Schematic arrangement of absorbers and trans- 
ducers in high-frequency tank. 

is available for special tests. The cylinders fit the 
standard transducers and the plate is of a size to fit 
across the guides in the test area. 

Reverberation Control. There are a number of fac- 
tors which influence the reverberation in a tank. 
Among these are the size and nature of the walls and 
the directivity pattern (or beam width) of the projec- 
tor. It is fairly obvious that a narrow beam is much 
easier to control than is a broad beam that spreads 
out and strikes the walls of the tank close to the pro- 
jector. 

Since at these high frequencies the beam widths are 
relatively narrow, it was decided to place absorbing 
units around and behind the hydrophone only, in 
such a manner that the portion of the sound beam 
that passes the instrument will strike the absorbers 
and be reduced to a negligible intensity. The particu- 
lar arrangement used is a cluster of 21 cylindrical 
absorbers grouped as shown in Figure 44. The pro- 
jector is beamed at the hydrophone from the opposite 
end of the tank. The tank was made elliptical with 
the thought that with the projector at one focal point, 
any stray radiation would strike the walls and pass 
through the other focal point. If the hydrophone 
were then placed about 4 inches in front of the focus, 
the stray radiation would have to pass through the 
absorbers before striking it. However, it was found 
in practice that this action contributed little to the 
reduction of reverberation and that the projector 


EQUIPMENT AT MOUNTAIN LAKES 


111 


could be placed anywhere in the tank as long as it 
was beamed at the hydrophone and absorbers. 

Each absorber unit^^ is a hollow cylinder of acous- 
tic rubber 4 inches in diameter and 46.4 inches long, 
packed with 29 ounces of No. 00 mesh steel wool and 
filled with deaerated castor oil. 

The absorber units are sufficiently rigid to stand 
unsupported in water. With the particular stacking 
arrangements used, the sound must travel through 
the equivalent of four such units before reradiating 
into the tank. The characteristics of one absorber are 
shown in Figure 45. The insertion loss for frequencies 
above 1 me was beyond the sensitivity of the measur- 
ing system and transducers. 

Two types of standard transducers are used with 
this system. One set, with the active face 3 cm in 
diameter, has an exceptionally narrow beam, and as 
a result will go down to 80 kc before reverberation 
affects the results. This type of unit is used up to 800 
kc. The diameter of the second type is 1 cm and it is 
satisfactorily used as far as 2,200 kc. Its directivity is 
such that reverberation interferes below 300 kc. 

One method of minimizing the effects of reverbera- 
tion is pulsing, which is used with success on the in- 
termediate-frequency systems but has not been tried 
with this system. The only reason was lack of time. 
The required modification is not difficult and the 
benefits to be derived should be as great as in the 
other systems. Another method that was tried utilizes 
acoustic lenses. (See reference 57.) The idea behind 
the use of such lenses was that, if the beam from a 
small projector could be focused by the lens on the 
hydrophone, the ratio of signal-to-reverberation in- 
tensity should be raised considerably— high enough 
in most cases to give an accurate evaluation of the 
sound field. 

A lens 10 centimeters in diameter was constructed 
of polystyrene for operation at 150 kc. A projector 
was set at 25 cm from the lens center and a hydro- 
phone at the conjugate focus 125 cm on the opposite 
side. Corrections were applied for spherical aberra- 
tion and thick-lens effects, and the coincidence of 
center of curvature and focus was avoided to prevent 
standing waves. The theory of geometrical optics can 
be used in designing sonic lenses and if due care is 
taken the actual lenses will perform according to the 
theory. For the lens described the increase in signal 
strength at the focus was calculated to be 1 1 db for the 
1-cm projector and 8 db for the 3-cm projector. These 
values were obtained experimentally. It should be 



0 200 400 600 000 1000 

FREQUENCY IN KC 

Figure 45. Insertion loss of one sound-absorbing unit. 

mentioned that lenses can be used in making reci- 
procity calibrations, if a properly modified parameter 
is used. 

One objection to the use of lenses is that the sound 
velocity in them changes considerably with frequen- 
cy. This is to be expected from the analogous optical 
dispersion, but it adds to testing the difficulty that 
the instruments which are in focus at one frequency 
are not at another. To eliminate dispersion, various 
shapes of reflectors may be used. These behave as 
expected, but the problem of preparing and main- 
taining surfaces sufficiently smooth for the high fre- 
quencies is too difficult. Another objection to lenses 
is that the reflections are excessive in some cases. In 
addition, the lenses are inconvenient to mount and 
modified parameters must be used in the calculations. 
All in all, the use of more directive beams and sound 
absorbers is found to be the most effective. 

Standing Waves. Another problem in all acoustical 
measurements is the presence of standing waves. Even 
with no reflections from the boundaries of the me- 
dium, standing waves may still occur between projec- 
tor and hydrophone, or simultaneously between each 
instrument and a third object in the sound field. In 


112 


USRL TEST STATIONS 


100 TO 800 KC HEAD {WITH 
STORAGE COVER REMOVED) 




COAXIAL CONDUIT PIPES 



300 TO 
2200 KC HEA[ 


COUPLING NETWORK HOUSING-J'HYDROPHONE PRE-AMPLIFIE 

WITHDRAWN FROM HOUSING 




Figure 46. High-frequency transducers. 


one measurement on lenses, standing waves were 
generated between the projector and the first face of 
the lens and between the lens’ faces. 

It can be shown that, if a condition for interfacial 
standing waves exists and a frequency sweep is made 
with a sinusoidal source of sound, the response of the 
hydrophone as a function of frequency will be com- 
posed of a series of alternate maxima and minima. 
I'he frequency difference. A/, between maxima and 
minima may be shown to be A/ = c/2d, where c is the 
velocity of sound and d is the distance between the 
reflecting surfaces. It follows that, if the sound source 
is composed of a band of frequencies greater than A/, 
the standing waves will be broken up. This is the pur- 
pose of the noise generator mentioned previously. 
On the basis of a 6-kc band for this noise generator, 
the minimum distance that can be tolerated is 12.5 
cm. Actually, this figure must be modified by the 
fluctuation allowed in the recorded data. Another 
method of eliminating standing waves is to separate 
the instruments until the normal acoustical losses 
reduce the reflections to a negligible intensity. 

It is important to note that the above expression 
for A/ provides an excellent analytical tool for cer- 
tain observations and gives a basis for analyses of the 
geometry which is involved in a particular experi- 
mental set-up. 


Standard Transducers. While the transducers have 
been described in detail elsewhere,*’’" it is best to point 
out some of their novel features which are definite 
aids in making measurements at these frequencies. 

The instruments consist of three sections: (1) sound 
piston and coaxial leads, (2) network housing, and 
(3) networks. Any one of the three parts is inter- 
changeable with parts from other instruments. The 
connections between parts are kept watertight by 
rubber gaskets. The tops of the pipes containing the 
coaxial leads have coaxial jacks into which fit coaxial 
plugs on the bottom of the network assemblies, and 
the assemblies have coaxial jack outputs for connec- 
tions to the junction boxes. These connections are 
made by means of coaxial patch cords. 

The networks serve the purpose of matching the 
transducers to the transmission lines. There are two 
such networks available; one is a hydrophone pre- 
amplifier, and the other is a transformer for use with 
the projector. The two coupling networks have been 
described. 

Electroacoustical Measurements 

The technique of measurement with the high-fre- 
quency system parallels to a great extent those used 
in the lower frequency systems. However, the fre- 
quencies and wave lengths involved necessitate a 


EQUIPMENT AT MOUNTAIN LAKES 


13 


more exact control of some of the electric and acous- 
tic parameters. These are treated in detail in the fol- 
lowing discussion. 

Adjustment and Maintenance of Electric System. 
After the electric system is in thermal equilibrium, 
the frequency scale on the oscillator must be set. As 
mentioned previously, this is done at only two points, 
76 kc and 2 me, and the scale is then assumed to be 
correct over the rest of the range. 

The output level is determined by connecting the 
oscillator (or power amplifier, if used) to the trans- 
mission measuring set and finding the power dis- 
sipated in 72 ohms. The set is calibrated to read a 
specific power level of 133 db vs 10“^^ watt, but the 
introduction of standardized attenuators between 
the source and the set allows the measurement of 
higher powers. The power level adjustment for the 
oscillator may be made at any frequency, since its 
output is flat within 0.15 db over the entire range. In 
the case of the power amplifier, which does not have 
a flat response, the adjustment is usually made at 50 
kc. 

The carrier frequency and modulation balance of 
the detector are then adjusted. The sensitivity of the 
receiving system (detector plus recorder) is adjusted 
at a specific frequency, usually 50 kc, since the detec- 
tor does not have a flat response. This adjustment is 
made with the fine gain control so that, with the gain 
dials of the detector set at zero, the recorder reads 
directly the input level to the detector. The adjust- 
ment results in a full-scale deflection of the recorder 
for an input level to the detector of 135 db vs 10“^® 
watt. The level of any recorded signal is, then, the 
recorded level minus the gain of the detector as in- 
dicated by the dial settings. 

With these adjustments, the equipment is ready 
for use but checks of this nature must be made several 
times each day to correct for minor changes which 
may have occurred. 

The fact that the power amplifier and the detector 
do not have flat frequency characteristics entails no 
serious difficulty as long as the variations are rec- 
ognized and corrections made. In most acoustical 
measurements, comparisons are made between an 
unknown and a standard. If the same equipment is 
used with each instrument, the ratio between the two 
response records is correct, although the actual read- 
ings are not. Only the measurement and interpreta- 
tion of specific quantities, such as current to a 
projector, require consideration of the frequency. 


Calibration of Standards. The principle of reci- 
procity (see Chapter 5) was used in calibrating the 
high-frequency standards. The actual testing pro- 
cedure has already been described, so that only the 
precautions peculiar to this range will be treated 
here. 

With the short waves and narrow beams involved, 
the hydrophone must be accurately oriented with 
respect to the projector and both must be rigidly 
clamped, as a very small movement may introduce 
serious errors. Care must also be taken to have the 
projector beam completely cover the active acoustic 
face of the hydrophone. It is unfortunate that, in 
many high-frequency units, the beam tends to 
wander from the established acoustic axis as the fre- 
quency is varied. This will introduce serious errors 
unless the hydrophone is located far enough from the 
projector to have only negligible variations in the 
portion of the field being measured. This variation 
in beam pattern may be detected by reorienting the 
projector at various frequencies and noting the 
change in direction of the axis. Effects such as these 
necessitate the complex positioning equipment. 

The orientation of the projector and hydrophone 
is made at the highest possible frequency because 
of the increased accuracy afforded by the sharper 
beam pattern. However, the possibility of beam 
wandering must always be taken into consideration. 

The units should be far enough apart so that cor- 
rections for spherical waves are unnecessary. (See 
Chapter 5.) The method of testing for this effect, and 
also for that of incomplete coverage of the hydro- 
phone face by the sound field, is to take response runs 
at several distances. The shortest distance at which 
the inverse distance law holds determines the mini- 
mum testing distance that should be used. 

To date, no instrument tested in the tank has 
necessitated spherical wave corrections. Such an in- 
strument could be calibrated in the pier test areas 
which allow a much larger testing distance. 

If standing waves are present between transducer 
faces, they can, in some instances, be eliminated by 
rotating the face of the hydrophone through a small 
angle. 

In measuring the current to the projector at these 
frequencies, special precautions have to be taken be- 
cause of the stray capacities involved. The circuit is 
shown in Figure 47 and it is to be noted that both 
leads are isolated from ground. 

Even though the ground between resistor and 


114 


USRL TEST STATIONS 



Figure 47. Circuit schematic for current measurements 
in unbalanced circuit. 


transformer is removed, the circuit cannot be used 
for a projector with one side grounded because the 
current flowing through the stray capacity of the coil 
and leads will be included in the reading. For the 
same reason the circuit for the intermediate-frequen- 
cy system shown in Figure 48 cannot be used. 

Calibration of Unknoiun Instruments. When a set 
of calibrated standards is available, the calibration of 
unknown units is fairly straightforward. A known 
sound field is established between a standard projec- 
tor and hydrophone. The latter is replaced by the 
unknown, the response of which is found in the same 
field. It is assumed that the unknown will satisfy the 
acoustic restrictions of proper beaming, absence of 
standing waves and reverberation, and the use of a 
testing distance that will not require correction for 
spherical waves. The sensitivity of the unknown is 
determined by comparison with the response of the 
standard, but all coupling losses are to be accounted 
for. If the unknown is a projector, a standard hydro- 
phone is used to measure the sound field it generates 
for a given current or for given power. 

Occasionally a hydrophone is found with only one 
high lead and the case grounded. A coupling loss 
measurement may be made on this instrument by im- 
mersing it in a glass container of water which insu- 
lates it from ground. However, care must be taken 
to avoid standing waves between the transducer head 
and the walls of the container. 

Another case requiring special treatment is the 
measurement of the current to a high-impedance 
projector with unbalanced electrical connections. If 
the water resistance is high enough, the method out- 
lined previously will give fairly accurate results if the 
case-to-system ground is removed. If the water resist- 



MEASURING 

SYSTEM 

Figure 48. Circuit schematic for current measurements 

in balanced circuit. 

ance is not sufficiently high, recourse must be had to 
measuring the impedance of the projector, driving 
it from a known voltage and calculating the current. 
If the impedance of the projector is high enough, the 
transmission line can be terminated with a 72-ohm 
resistor and the projector connected across it. The 
voltage across the projector can be calculated then 
from the current through the resistor. 

Greater stress has been placed upon measuring cur- 
rent than upon calibrating for a given available 
power. There are several reasons for this. The main 
reason is that the transformers used in coupling pro- 
jectors to the line do not act like ideal transformers 
over this frequency range and hence cannot be repre- 
sented by a voltage source in series with a resistance. 
This immediately destroys the concept of available 
power. Likewise, the power amplifier output does 
not conform to such a representation and so will not 
satisfy the conditions involved in the definition of 
available power. The use of a resistance pad, which 
would eliminate the impedance variations, would 
increase tremendously the size of the power amplifier, 
if the same power output were to be preserved. In 
addition to this, the problem of providing pads for 
various projector impedances for this frequency 
range is tremendous. As a result, all calibrations of 
projectors are at the moment made on a current basis. 

The actual recording of data for all such acoustic 
tests follows quite exactly the procedure and printed 
forms used on the lower frequency systems. 

Frequency Scaling 

The high-frequency system is admirably adapted 
to perform tests on scale models, with the wave length 
of the sound shortened on the same scale. Even meas- 


EQUIPMENT AT MOUNTAIN LAKES 


115 



Figure 49. Application of high-frequency system in testing scale model submarine in outdoor test area. 


urements of the reflection coefficient of such models 
will give a value for the reflection coefficient of the 
actual target. As the dimensions of the target and 
the wave length of the sound in actual operations are 
both reduced in the same ratio, all the reflection and 
diffraction of sound from the scale model and from 
the actual craft will occur in exactly the same 
manner. 

Figure 49 shows a scale model of a submarine with 
all linear dimensions reduced 60 to 1. Measurements 
of the reflection coefficient carried out at 1565 kc 
give the results which would be obtained for similar 
measurements on the actual object at 26 kc and dis- 
tances 60 times those used for the model. Sound re- 
flected from the model is measured by an adjacent 
hydrophone shielded from the direct radiation of the 
projector. As the model is placed at several distances 
from the projector and rotated around various axes 


of symmetry, measurements of the reflection coeffi- 
cient are obtained as functions of the range and the 
aspect of the model relative to the projector. 

® ^ ^ Low-Frequency Pressure System 

Iffie low-frequency j^ressure system at the Moun- 
tain Lakes laboratory covers 2 to 100 c and was 
evolved as a result of the increasing need for an ac- 
curate method of hydrophone calibration at these 
frequencies under controlled conditions of tempera- 
ture and pressure. The system was designed and con- 
structed by the Bell Telephone Laboratories, Inc., 
under NDRC contract.^^ 

Uses and Limitations 

The testing technique with this apparatus is inde- 
pendent of auxiliary hydrophone standards. The 



116 


USRL TEST STATIONS 


tank consists of a rigid closed cylinder, 10 inches in 
diameter and 20 inches high, in which the test hydro- 
phone is hung. A coil-dri\en diaphragm from an 
XDRC IK projector is mounted in the chamber wall 
and produces high sound pressures of known magni- 
tude in the water-filled chamber. The sound pressure 
is dependent mainly on the force factor of the sound 
source and on the mass and stiffness reactances of the 
diaphragm and the enclosed neater. The chamber and 
the circular sound source and mounting are shown 
in Figure 51. I'he chamber and source together meet 
the rec|uirements of a stiffness-controlled system op- 
erating over a fairly wide range of low frequency. The 
limitations of the chamber and the associated elec- 
trical circuit are such that the effective range over 
which accurate calibrations of “hard” (essentially in- 
compressible) hydrophones may be made, lies be- 
tween 2 and 100 c. Calibrations can be made only on 
hydrophones which are sufficiently hard not to lower 
the system stiffness appreciably. Reduction of the sys- 
tem stiffness lowers the resonant frequency and, con- 
sequently, the upper limit of the frequency range 
through which calibrations may be made. 

Facilities are incorporated to obtain measurements 
at temperatures between 35 and 100 F and at pres- 
sures up to 100 pounds per sq in. The use of a closed 
testing chamber permits measurements of the de- 
pendence of a hydrophone upon temperature and 
pressure to be made more con\eniently than under 
free field conditions. Approximately six hours are re- 
quired to obtain a characteristic frequency response 
o\ er a complete temperature cycle at each hydrostatic 
pressure. 

Hydraulic System 

A schematic diagram of the hydraulic system ap- 
pears in Figure 52. 

Calibration Chamber. 7'he chamber consists of two 
bronze castings 1 inch thick, together weighing about 
500 pounds. The dimensions and construction are 
shown in Figure 53. The strength and size are such 
that no chamber or wall resonances occur below 200 
c. I'he resonant frequency of the sound source is well 
above 250 c. 

Since even small quantities of air greatly reduce the 
chamber stiffness, provisions have been made for de- 
aerating the water by reduced pressure and also for 
air venting of the chamber. The shape of the chamber 
was designed to facilitate the removal of air, and the 
direction of the water flow is such that air is carried 



Figure 50. Electrical equipment of low-frequency system. 



Figure 51. Calibration chamber section of low-frequency 
system tank, showing sound source and other details. 






117 


EQUIPMENT AT MOUNTAIN LAKES 



Figure 52. Operating schematic of hydraulic arrangement for lo\\'-fre(|uency system. 


to the venting cock at the top. When the system is 
freshly filled, the water may be heated and circulated 
under vacuum to decrease the amount of air in solu- 
tion, though the chamber and instrument are care- 
fully debubbled by hand before each test. 

Hydrostatic Pressure Chamber. Hydrostatic pres- 
sures up to 100 pounds per square inch are obtained 
by the use of air from a small compressor unit. The 
pressure is transmitted through a moulded rubber 
bag of negligible stiffness, mounted in a bronze 
chamber. As may be seen in Figure 52, the water side 
of the bag connects to the calibration chamber 
through a line with high acoustic impedance. The 
air line connects to both the pressure chamber and 
the rear chamber of the sound source through suit- 
able control valves, thus equalizing the pressures on 
the sound source diaphragm. A manometer is con- 
nected to the system in such a way that the pressure 
differential may be checked at all times. Since there is 
need during a hydrophone calibration to read the 
mercury column to within itO.l mm, an optical sys- 
tem was used which allows displacements of less than 
1 cm to be read within ±0.05 mm without parallax. 



Figure 53. Calibration section of low-frequency system 
tank, showing top lifting mechanism anti major di- 
mensions. 



18 


USRL TEST STATIONS 


Equipment for Cooling, Heating, and Circulating 
the Water. The water line from the test chamber 
leads to a 13x25-inch pressure tank in which are 
sealed the impeller of a circulating pump and the 
evaporating coils of a cooling system. A 2,000-watt, 
110-volt immersion heater is located in the line re- 
turning from the tank to the test chamber. Control 
facilities enable the system to be held within 1 degree 
of any desired temperature. 

Temperature reduction is accomplished by an air- 
cooled Freon compressor with motor. A close 

control of temperature in cooling is obtained by us- 
ing the heater in conjunction with the refrigerating 
unit. Safety controls are incorporated which prevent 
the system from either freezing or rising above 105 F. 

Circulation is maintained by a rotary pump of 
hp while the temperature is being changed, but when 
the desired value is reached the circulation is stopped 
and the test chamber closed off by valves. 

The water is deaerated by a vacuum pump with a 
%-hp motor. 

Electrical System 

The apparatus consists of an oscillator, power am- 
plifier, variable attenuator, and receiving amplifier 
and level indicator. Figure 54 shows the connections 
in a typical testing setup. The units which are per- 
manently installed are mounted in bays adjacent to 
the hydraulic system. For flexibility and ease of op- 
eration, the equipment is terminated in several jack 
strips. 

The signal generator is a Hewlett Packard audio 
oscillator. Model 202DR. It imposes the lower limit 
of 2 c on the system. Sound pressures up to 10^ dynes 
per sq cm may be used at any point in the frequency 
range. 

The power amplifier was designed especially for 
low frequency. It operates on alternating current and 
has a maximum output of 6 watts. The gain may be 
controlled over 45 db in 5-db steps with continuous 
adjustment through each step. The circuit has been 
equalized to give a frequency response which is flat 
to 0.1 db over the working range. The input and out- 
put impedances are 600 ohms and 50 ohms, respec- 
tively. 

The attenuator provides 45 db in 1-db steps giving 
a dial which reads hydrophone responses directly 
from —80 to —125 db vs 1 volt per dyne per sq cm 
when the chamber stiffness is 10^ dynes per cm. 

The component parts of the receiving amplifier 



Figure 54. Electrical measuring circuit used with low- 
frequency system. 


and indicator are the amplifier, a copper oxide recti- 
fier, and a meter reading from — 10 to + 10 db. Input 
impedances of 80,000 ohms unbalanced, and 600 
ohms balanced or unbalanced, are available. The 
gain is essentially flat over the range of 2 to 100 c. 
The dial of the amplifier-gain control covers 50 db in 
5-db steps so that its setting plus the meter reading 
gives the input signal level into the 600 ohms in db 
vs 10-1® watt. With a fixed 20-db gain which may be 
added, a range from 50 to 120 db can be read directly. 

Two combinations of capacitors and resistors are 
available for insertion in the meter circuit. For fre- 
quencies up to 10 c, the one with the larger time con- 
stant is selected to give a fairly steady meter deflection. 
At higher frequencies, the lower time constant is 
chosen to increase the rate of meter response. 

Procedure 

The stiffness of the chamber must be measured 
over the whole range of both pressure and tempera- 
ture since it enters as a correction in all determina- 
tions of hydrophone sensitivity. The procedure is to 
close key 1 (Figure 54), thus putting a small direct 
current through the projector coil and producing a 
pressure indicated on the manometer. From this 
measurement and known diaphragm constants, the 
stiffness may be computed. Extensive computations 
are obviated by the use of a chart relating these fac- 
tors. 

The system is thus calibrated so that the sound 
pressure in the chamber is known for any projector 
current at any point in the range of stiffness. Thus 
the sound pressure which produces the measured out- 
put voltage of the hydrophone could be calculated 



119 


EQUIPMENT AT MOUNTAIN LAKES 


from the projector current but the circuit is arranged 
to save the calculation and calibrate the hydrophone 
in absolute units. With key 2 in one position, the 
meter is connected to the hydrophone and deflected 
by its output voltage. With the key reversed, the me- 
ter is across a resistor carrying the projector current 
which produces the sound field. The attenuator is 
then adjusted until the meter reads the same as when 
connected to the hydrophone. As the position of the 
attenuator is related to the sound field and its output 
voltage matched to that of the hydrophone, it may be 
calibrated to read the sensitivity of the hydrophone 
directly in units referred to 1 volt per dyne per sq cm 
for the normal tank stiffness of 10'^ dynes per cm. Cor- 
rections for other values of the stiffness are taken from 
a chart. 

High-Power System 

The high-power amplifiers at the Orlando and 
Mountain Lakes stations have approximately the 
same electric characteristics. The overall gain of each 
amplifier is about 24 db and the response is flat with- 
in 1.5 db from 1 to 100 kc. At the extreme ends the 
response falls off, 4 db at 150 kc, 10 db at 200 c. The 
amplifiers should not be driven at high powers below 
200 c because of increased coil losses. 

The output impedance is 100 ohms; the input may 
be either 135 or 600 ohms. The power available with 
a minimum of distortion is 1,200 watts but 1,500 may 
be obtained with a slight increase in harmonic 
content. 

Each system is provided with a repeating coil that 
will match the line of impedance of 100 ohms to a 50-, 
100-, or 500-ohm load. In addition, there is a 30-db 
pad capable of dissipating 1,500 watts and having a 
100-ohm input and a 135-ohm output to match the 
repeating coil to the transmission measuring set. At 
both stations the pads and repeating coils are 
mounted in the transmitting booth and the connec- 
tions appear on a coaxial jack strip in the booth. 

It is obvious that care has to be exercised in the 
choice of lines and switching elements for currents 
that may reach 6 amperes and potentials as high as 
500 volts. It has been found that the lead-covered co- 
axial pier lines and the coaxial jacks handle these 
powers satisfactorily but special patch cords had to 
be constructed to provide connections of adequate 
capacity. The plan of using patch cords and jack 
strips at these powers involves the danger of discon- 


necting the high-level side when in use. Large arcs 
may result and the damage to the jacks and terminals 
will be minor compared to that which may occur 
from the overload voltage developed in the final stage 
of the amplifier. With this in mind, particular atten- 
tion is given to the location of the jacks, and special 
lines are run whenever possible. Warning signs are 
kept at the critical junction points. 

The connections from the intermediate-frequency 
systems to the low-level input of the power amplifier 
are made in a jack field associated with the amplifier. 
No caution is needed for these connections, as break- 
ing them under load causes no damage. 

In using the amplifiers, power-level measurements 
are made in two ways. The first involves a 30-db pad 
and the transmission measuring set to determine the 
available power for the 100-ohm output. The second 
requires either a thermocouple wattmeter or a record- 
ing wattmeter to measure the power delivered to the 
load. Apart from the precautions needed for the high 
powers involved, these amplifiers are treated merely 
as extensions to the existing equipment. 

In their electrical details the amplifiers at Orlando 
and at Mountain Lakes differ considerably. While 
both are essentially two-stage, push-pull, transformer- 
coupled units, the power requirements are quite dif- 
ferent. 

The Orlando amplifier is a Navy echo-ranging type 
in which the input, output, and interstage transform- 
ers are replaced with special transformers designed 
for the particular frequency and power range. How- 
ever, the power supply and control circuits are re- 
tained and these require a 3-phase, 440-volt, 60-cycle 
supply. Since this is not available from the power 
lines, a motor-generator set was used with 6-kva maxi- 
mum output. 

To compensate for fluctuation of line voltage, in- 
ternal impedance of the generator, and other sources 
of instability, an electronic regulator is used which 
maintains the peak voltage within 1 per cent. Con- 
trol of the peak is chosen because the critical voltages 
in the amplifier are determined by the peak of the 
supply rathef than by the rms value. Because of the 
change in wave form, the rms value shows a variation 
of 5 per cent from no load to full load for a peak varia- 
tion of only 1 per cent. Another reason for controlling 
the peak voltage is the speed of response which in this 
regulator will compensate for any changes in a few 
cycles. Adjustments are also made to prevent hunt- 
ing. Such adjustments are very necessary with rotat- 


120 


USRL TEST STATIONS 




Figure 55. Electrical equipment of high-power system. 


ing machinery of this size in order to realize the point 
of greatest stability. 

The Mountain Lakes amplifier operates at 220 
volts directly off the single-phase, 6()-cycle supply. It 
differs from the Orlando unit in that all grid-supply 
voltages are regulated quite closely since the grid 
currents may rise to 20 or 30 milliamperes in the final 
power stage. 

The amplifier at Mountain Lakes is shown in Fig- 
ure 55. I’he bay appearing at the side is used for ter- 
minating crossties to the other systems and for con- 
nections to the amplifier. In this bay are also a master 
intercommunication station, attenuators, and a sepa- 


rate signal source (AVTstern Electric Company 17B 
oscillator). Plans are being considered to incorporate 
in this bay a generator and transmitter modulator 
with longer pulses than are now available. This 
pulsing equipment would be used with the power 
amplifier to simulate actual operating conditions in 
echo-ranging systems. The arrangement would allow 
projector characteristics to be measured under typi- 
cal working conditions. This is of particular impor- 
tance in the study of cavitation and of heating due to 
power losses on the transmitting efficiency. 

^ High-Pressure System 

Measurements on transducers operating in the in- 
termediate-frequency range and under hydrostatic 
pressure up to 300 lb per sq in. may be made in the 
high-pressure tank. The pulse system is used to over- 
come the difficulties of testing in a confined medium 
but there are still limitations as to what can be tested 
in the tank. 

Description of High-Pressure Tank 

The tank, made of firebox steel, is a 

horizontal cylinder 8 feet in diameter and 14 feet 
long. There are eight glass-covered viewing ports 
along the sides of the tank. Two ports on top provide 
access to the interior. One port is oval in shape, being 
1 foot wide and 3 feet long. It is provided with a 21/2- 
inch thick steel cover. The other port is circular and 
has a diameter of 2 feet. Built as an integral part of 
the cover of this port is a single shaft rotator, similar 
to the rotator described in connection with the inter- 
mediate system. This rotator can be used in conjunc- 
tion with the polar recorder turntables of either 
intermediate-frequency system. The shaft of the 
rotator passes through a pressure gland in the center 
of the cover. The circular port is located 4i/2 feet 
from one end of the tank and the oval part is located 
3 1/2 feet from the other end. Baffles of 1-inch steel 
plate have been placed at the center of the tank as 
shown in Figure 61, leaving a square aperture 3 feet 
by 3 feet. An overhead monorail and hoist system is 
used to handle the heavy port covers and test instru- 
ments. 

Rails for a carriage from which to suspend trans- 
ducers are mounted in the tank under the oval port. 
1 he carriage is controlled by two threaded rods that 
pass through stuffing boxes in the end of the tank and 
terminate in hand cranks. One rod moves the car- 



EQUIPMENT AT MOUNTAIN LAKES 


121 



Figure 56. View of high-pressure lank, showing side viewing ports. 




Figure 57. View of oval port of high-))ressnre tank 


Figure 58. \’iew of circular port of high-pressnre tank 



122 


USRL TEST STATIONS 



Figure 59. View of high-pressure tank, showing top working area. 


riage lengthwise of the tank and the other rotates it 
about a vertical axis through a maximum of 30 de- 
grees. The travel of the carriage allows the distance 
between two transducers to be varied from 4 to 8 feet. 

Each port cover is held securely in place by four 
hydraulically operated wedges, constrained in the 
vertical direction by bridges. Each wedge, cut at an 
angle of 10 degrees, exerts a downward force of 

60.000 pounds on the cover when actuated by a 
hydraulic cylinder operated at a fluid pressure of 

1.000 pounds per square inch. The hydraulic system 
for operating the wedges is shown in Figure 60. The 
hydraulic system pump is equipped with a 1,000 
pounds per square inch automatic by-pass and can 
deliver 3 gallons per minute at this pressure. 

The tank is filled directly from the lake and, since 
it is drained frequently, no provision is made for in- 
hibiting the growth of organisms except that several 
chemical briquettes like those used in the high-fre- 
quency system are left in the tank to dissolve. The 


system for Oiling and applying pressure is shown in 
Figure 61. A pump with a capacity of 1,500 gallons 
per hour is used for filling the tank and, when neces- 
sary, for circulating the water through a heat ex- 
changer coupled to the heating system of the labora- 
tory. With all valves closed, the pressure is applied by 
a high-pressure pump governed by an adjustable pres- 
sure switch. The control is automatic and will keep 
the pressure at any value up to 300 lb per sq in. within 
± 5. Two safety valves protect the system from exces- 
sive hydrostatic pressures. 

The interior of the tank has been coated with the 
bubble layer developed by the Massachusetts Insti- 
tute of Technology in order to obtain some sound ab- 
sorption at the walls. 

Acoustic Measurements in High-Pressure 
Tank 

Acoustic measurements in the tank differ from 
those in a free field because of the relatively small size 


EQUIPMENT AT MOUNTAIN LAKES 


123 


of the tank and the absence of perfectly absorbing 
walls. The limitations imposed by the relatively short 
testing distances and the use of pnlsing to make the 
measurements independent of reflection have been 
discussed in Chapter 5. 

In actual testing, the optimum values of testing dis- 



PORT LOCKS CLOSE WHEN SIDES "A” ARE CONNECTED TO THE HIGH 
PRESSURE LINE AND SIDES "8" ARE CONNECTED TO THE RETURN LINE 

PORT LOCKS OPEN WHEN SIDES “B" ARE CONNECTED TO THE HIGH 
PRESSURE LINE AND SIDES'*A"ARE CONNECTED TO THE RETURN 
LINE 

Figure 60. Hydraulic system for holding port covers on 
high-pressure tank. 

HEAT EXCHANGER 
COUPLED TO 



Figure 61. Hydraulic system of high-pressure tank. Pumji 
No. 1 is used for filling tank and circulating water 
through heat exchanger. Pump No. 2 applies pressure to 
tank when all valves are closed. 


tance and pulse length being somewhat interdepend- 
ent within certain limits, the distance may be in- 
creased by using shorter pulse lengths and vice versa. 
Larger test distances may be used with transducers of 
good transient response than with sharply resonant 
ones. A pulse of 1.8 milliseconds is permissible when 
the test distance is 5 feet because of the baffles which 
prevent reflections from the sides of the tank. With- 
out baffles the reflections would be delayed only some 
0.8 milliseconds. At the maximum distance of 8 feet, 
the permissible pulse length is 0.4 milliseconds. For 
many transducers these distances and pulse lengths 
give results equivalent to those in a free field. Even 
when the tank will not permit such equivalent cali- 
brations, it should be possible to observe the relative 
performance of transducers as functions of tempera- 
ture and hydrostatic pressure. Observation of these 
functions is obviously not possible with tests made in 
the lake. 

The cleaning, rigging, and debubbling preparation 
of the transducers is identical with that for the free 
field testing, with the added precaution that the in- 
struments must be capable of operating at the desired 
test pressures. The available power is measured with 
the 30A set, with the transmitter modulator of the 
pulse system arranged for continuous-wave output. 
By the nature of the modulator circuit, the same in- 
stantaneous value of power is maintained when it is 
switched to the pulsing operation. 

Observation on the cathode-ray oscilloscope [CRO] 
allows rapid adjustment for the proper pulse length, 
delay time, and received pulse. The recurrence rate is 
set so the reverberation from one pulse does not inter- 
fere with the measurement of the succeeding pulse. 

6.2.7 Noise and Transient Measurements 

As pointed out in Chapter 5, acoustic noise may be 
classified, for the purpose of analysis, as continuous, 
such as thermal or cavitation noise, or as intermittent, 
such as waves of explosive origin. The method used in 
measuring continuous noise has been treated under 
the description of the intermediate-frequency system 
(15 c — 150 kc) in Section 6.2. None of the methods 
described previously is suitable for intermittent 
sound. 

Analysis and Measurement of Transient.s^‘1 

Transients may be considered as composed of sinu- 
soidal signals having a continuous distribution in 




124 


USRL TEST STATIONS 


Irequency and may be represented by the Fourier 
integral: 

£(/) = j Af sin (2 TT ft - af)df, (1) 

where and uj are the amplitude and phase of each 
component frequency and E (t) gives the time varia- 
tion of the resultant pulse amplitude and phase. For 
most practical work only the dependence Af on fre- 
quency need be determined. 

One method of obtaining this information about a 
transient is to make a record of its wave form and ana- 
lyze it. This technique may be used on high crest- 
factor noises which cannot be analyzed with the usual 
electric and recording systems. 

A schematic of the electric circuits for detecting 
and recording transient wave forms of high peak pres- 
sure is shown in Figure 62. A transducer is used to 
convert the acoustic pressure to an equivalent voltage 
which is amplified and impressed on a cathode-ray 
oscilloscope producing a beam deflection propor- 
tional at each instant to the acoustic pressure. By 
photographing the oscilloscope screen on a motion- 
picture film travelling at constant velocity, a record of 
the transient wave form in amplitude and duration is 
obtained. For accurate reproduction of the acoustic 
transient, the phase distortion and frequency discrim- 
ination must be kept to a minimum. 

A transducer with the XMX crystal head is selected 
primarily becavise of its small size and uniform fre- 
quency response. However, since this head has an 
x-CLit crystal of Rochelle salt, it is necessary that it be 
terminated in an impedance much higher than its 
own to minimize the effect of temperature. Because of 
microphonic effects, it is necessary to place all elec- 
tronic equipment a considerable distance away, pref- 
erably out of the water entirely. This involves long 
leads to the preamplifier. To prevent the hydrophone 
from becoming temperature-dependent because of 
the capacity of the connecting cable, a very small non- 
microphonic capacity (C in Figure 62) is connected 
in series with the crystal head at the junction of the 
head and connecting cable. This is in effect a capaci- 
tative voltage divider with an input impedance high 
compared with the XMX head. As a divider it has 
good phase and frequency characteristics as long as 
any resistive comjx)nents involved are high compared 
with the shunt capacitative reactance. It is also to be 
pointed out that any external capacity shunting the 



Figure 62. Scliematic of circuit used in measuriug 
transients. 


cable capacity will further increase the voltage divi- 
sion. The voltage-dividing effect of the network 
serves a second essential purpose in reducing the 
peak voltages resulting from high peak pressures to 
the point where they are on the linear portion of the 
preamplifier curve. The combination of crystal head, 
voltage-dividing network, and cable is treated as a 
unit and all acoustic calibrations are referred to the 
end of the cable. 

The preamplifier serves also as an impedance trans- 
former to provide a 100-megohm impedance to the 
hydrophone and 72 ohms to the transmission line. 
Two such preamplifiers are available, one with a gain 
of — 1 1 db for high sensitivity heads, such as the XMX 
type, and one with a gain of 35 db for low sensitivity 
heads, such as tourmaline types. Both have a flat re- 
sponse and a linear phase-frequency characteristic 
from 2 c to over 600 kc. For almost all measurements 
of transient phenomena, as well as those with high 
peak pressures, this laboratory has used the XMX 
head in conjunction with the first preamplifier. 

The oscilloscope used for these measurements is a 
Dumont type 247 provided with an external supple- 
mentary intensifier voltage for more brilliant traces. 

The records of the wave form were made on a mov- 
ing film camera (Western Electric Fastax) with film 
speeds up to 100 feet per second. Theoretically, fre- 
quencies as high as 600 kc may be resolved at this 
speed. The camera was used without the shutter 
mechanism. With the moving film, no horizontal 
sweep signal or transient sweep circuits are used in 
the oscilloscope but the vertical amplifier which mul- 
tiplies the voltage from the transient is essential. 

It is })erhaps best to point out here that, while all 
the electronic equipment was constructed with linear 
phase-frequency characteristics over the entire range 
of 2 c to 700 kc, the phase characteristics of the hydro- 
phones are not entirely known. In general, if a hydro- 
phone does not have any rising sensitivity with fre- 
(juency and if there are no “breakups” in the response 


EQUIPMENT AT MOUNTAIN LAKES 


125 



70 


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0 






TRIAL 

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SPE 

CTRL 

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>IS 

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^UL 

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trial: 

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60 


50 


40 


2 3 4 5 10 

FREQUENCY IN KILOCYLES PER SECOND 
B 


Figure 63. Transient analysis of explosive wave form. 


curve, it may be assumed that the phase relation is 
fairly linear. 1 his assumption should be further in- 
vestigated. However, the phase-frequency character- 
istic does not affect the amplitude-frequency analysis 
of the transient but only affects its wave form. 

I'he frequency analysis of the transient is made 
from the photographic record by the use of a har- 
monic analyzer. I’he USRL was fortunate in having 
the use of a Henrici Analyzer,'*^-"'’ through the coop- 
eration of the Department of Physics, Case School of 
Applied Science. Phis analyzer gives the relative am- 
plitudes of thirty harmonics. A complete frequency 
analysis of a transient requires from several hours to 
several days, depending on the complexity of the 
transient. I'he resultant data are then only relative 
and require siq^plementary computations for conver- 
sion to actual units such as pressure and frequency. A 
sample analysis of an explosive wave by this method 
is shown in Figure 63. The analysis gives the rms pres- 
sure in db vs 1 dyne per sq cm in a 1-c band as a func- 
tion of frequency. 


6.2.8 Auxiliary Laboratory Equipment 

Considerable auxiliary ecjuipment is needed in the 
calibration and maintenance of the measuring sys- 


tems as well as in the calibration of transducers. For 
convenience in description the apparatus is divided 
into six groups: resistance and impedance bridges, 
test meters, portable signal generators, cathode-ray 
oscilloscopes, wattmeters, and miscellaneous equip- 
ment. 

Resistance and fMPEDANCE Measuring Bridges 

The laboratories are pro\’ided with a number of 
admittance and impedance bridges suitable for meas- 
urements over a wide range of values with the various 
frequency ranges. 

The Western Electric Company 3A Impedance 
Bridge'^ is used in the frequency range 1 to 150 kc. ft 
is an admittance bridge of the comparison type, meas- 
uring impedance in terms of the equivalent parallel 
resistance and capacitance components, ft permits 
measurements on devices which are electrically bal- 
anced or unbalanced to ground, ft measures parallel 
resistance components up to 1,100 ohms directly with 
supplementary computation to 1 megohm. Parallel 
capacitance (or inductance considered as negative 
capacitance) may be measured directly up to 0.11 /xf, 
and above this value by the addition of external ca- 
pacitance. The bridge is designed for an overall ac- 
curacy in impedance determinations of ±0.5 per 
cent, but it has reduced accuracy for extremely high 
parallel resistive components, ft is ordinarily used 
with a 17B oscillator and a 31 A transmission measur- 
ing set detector described later in this section. A typi- 
cal arrangement of these devices can be seen in Figure 
4. 

The Bell Telephone Laboratories W-10134 Imped- 
ance Bridge consists essentially of two units, a capac- 
itance comparison bridge and a Maxwell inductance 
bridge, thereby obviating the computations necessary 
to convert parallel values to series values or vice versa. 
The use of the bridge is limited to electrically unbal- 
anced instruments and a frequency range of 200 c to 
1 50 kc. 1 he comparison bridge measures capacitances 
from 0.1 fx^ii to 1.11 ^f and conductances from 0.01 
/xinho to 111,100 /xinhos. The Maxwell bridge meas- 
ures inductances from 0.1 /xh to 1.1 1 h and resistances 
from 0.01 ohm to 111,100 ohms. I he accuracy of 
direct bridge readings is approximately ± 1 per cent. 
Complete operating instructions and descriptive ma- 
terial are available which give correction factors to 
obtain a precision of ±0.1 per cent. 

To facilitate impedance measurements at the test- 
ing area, this bridge, together with a 17B oscillator 


126 


USRL TEST STATIONS 



Figure 64, Making impedance measurements using the 
W10134 impedance bridge, the 31A detector, and the 17B 
oscillator. 


and a 31 A transmission measuring set, is mounted on 
a rack equipped with casters. See Figure 64. 

The W-10093 Capacitance and Conductance Bridge 
for measurements at high frequencies was constructed 
by Leeds and Northrup Company, Inc. in accordance 
with a design by Bell Telephone Laboratories. The 
normal frequency range is 50 kc to 5 me but it may be 
used as low as 10 kc and as high as 10 me without 
serious loss of sensitivity or accuracy. The capacitance 
range is from 0.01 to 1,100 /x/xf either positive or nega- 
tive and the range may be extended well beyond 
11,000 /x/xf by means of five plug-in standards. The 
conductance range is from 0.001 to 1,100 ^mhos and 
may be extended to 1 1 ,000 /xinhos and further by 
plug-in standards. Both types of measurement may be 
extended even further by connecting a known admit- 
tance in series with the one to be tested. For the nor- 


mal frequency range the accuracy is about 0.25 per 
cent. 

The General Radio Company Impedance Bridge 
Model 650 A is a direct-reading instrument with a self- 
contained battery and a 1,000-c oscillator. It gives 
quick approximations of impedance at 1,000 c or 
with direct current. It gives d-c resistances from 1 
ohm to 1 megohm, capacitances from 1,000 fifii to 
100 /xf, and inductances from 1 iih to 11 h. 

A Leeds and Northrup Company Wheatstone 
Bridge is used for precise d-c resistance measurements. 

Test Meters 

The meters used at the laboratories are standard 
ohmmeters and voltmeters with the exception of a 
special megohmmeter and a 31 A transmission meas- 
uring set. Several portable Simpson and Weston volt- 
ohm ammeters are used in the maintenance of the 
electronic equipment and in making the customary 
electrical checks on transducers. A number of elec- 
tronic voltmeters are used for measurements where 
the test meter must have a high impedance. 

A Ballantine a-c Voltmeter measures rms voltages 
from 0.001 to 100 volts over a frequency range of 
about 10 to 150,000 cycles with a general accuracy of 
2 per cent. When used in conjunction with a decade 
amplifier, measurements may be made considerably 
below a millivolt with frequencies from 10 to 100,000 
c. The use of a special multiplier extends the upper 
limit to 1,000 volts. 

The Hewlett Packard Vacuum-Tube Voltmeter, 
Model 400 A, is used extensively for relative measure- 
ments of rms voltages between 0.03 and 300 volts over 
the frequency range 10 c to 1 me. 

The Measurements Corporation Electronic Volt- 
meter, Model 62, provides ranges of 1, 3, 10, 30, and 
100 volts with an accuracy of 2 per cent of the full- 
scale reading. When used with its associated probe, it 
allows measurements from 30 c to 150 me. It should 
be noted that it reads peak voltages. A specially de- 
signed vacuum-tube thermocouple voltmeter was 
constructed by USRL for rms voltages. 

A special Rawson Electrical Instrument Company 
Megohmmeter is used primarily for measuring in- 
sulation resistance at 2,500 volts and covers 0 to 100 
and 0 to 10,000 reading directly in megohms. 

A Western Electric Company 31 A Transmission 
Measuring Set is used extensively as a null indicator 
for the various impedance bridges. The high sensitiv- 
ity and frequency discrimination provide excellent 



EQUIPMENT AT MOUNTAIN LAKES 


127 



Figure 65. W 10093 capacitance and conductance bridge 

for use at high frequencies. 

resolution in balancing under severe conditions of 
wave distortion. The set consists of an amplifier with 
calibrated gain controls, an oscillator and modulator 
circuit operating in conjunction with a 20-c band-pass 
crystal filter, an output meter, and a regulated d-c 
supply circuit operated on 1 15-volt, 60-c power. The 
set has been designed to operate through the fre- 
quency range 1 to 150 kc with switches to permit its 
use as a wide-hand or a sharply tuned instrument. 
Continuous tuning control from 10 to 150 kc is ef- 
fected by the frequency adjustment of the local 
oscillator.^- 

PoRTABLE Signal Generators 

The Western Electric Company 17 B Oscillator is a 
heterodyne type operating on 60-c power and deliver- 
ing levels up to 160 db vs 10“^^" watt into a 135- or 
600-ohm load and over a frequency range of 1 to 150 
kc. Below 1 kc the power output decreases and the 
wave form becomes poor. It is used extensively in im- 
pedance measurements because of its highly stable 
and uniform output level, accurate frecpiency calibra- 
tion, and low harmonic distortion. 

The Hewlett Packard Oscillator, Model 200-Cr 
covers a frequency range of 20 c to 200 kc. It is de- 
signed to deliver a signal level of 150 db vs lO-i^'^ watt 
into a 1,000-ohm resistive load, although its output is 
not critically affected by the loading. The total har- 
monic distortion, under proper operation, is less than 
1 per cent. 


The Measurements Corporation Square Wave Gen- 
erator, Model 71, permits rapid determination of the 
phase and frequency characteristics of many types of 
amplifiers and networks when used in conjunction 
with a cathode-ray oscilloscope. The time of rise on 
the wave is about 0.2 microsecond with frequencies 
from 5 to 100,000 c. The Fourier analysis of these 
waves shows components that give a resultant range 
of investigation from 1 c to several megacycles. 

T he Dumont Electronic Switch and Square Wave 
Generator, Type 115A, produces square waves at 10 
to 500 c with a form that reaches full amplitude with- 
in a few microseconds. In addition to its usefulness in 
studying the performance of amplifiers and other net- 
works, it may be used in conjunction with a cathode- 
ray oscilloscope to make comparison studies of ampli- 
tude, wave form, phase, and frequency between two 
electric signals. 

Cathode-Ray Oscilloscopes 

Practically all measuring circuits require cathode- 
ray oscilloscopes. Dumont Type 175 A is used to pro- 
vide visual monitoring in each of the 15-c to 150-kc 
test systems. A Dumont Type 247 is used with an 
auxiliary circuit supplying higher accelerating poten- 
tials for special studies of transients requiring high- 
speed oscillograms. 

A Renier Model 556 operates at frequencies up to 
4 me and is usefid for observing wave forms at the 
higher frequencies and for observing carrier modu- 
lator balance. 

Wattmeters 

A thermocouple wattmeter and a recording-type 
wattmeter have been developed by the laboratories 
for the measurement of power over the large fre- 
quency range used in underwater acoustics. The two 
operate on different principles and the second was 
particularly designed to be used in conjunction with 
the present electrical systems. 

Thermocouple Wattmeter As the circuit of the 
thermocouple wattmeter shows in Figure 66, the in- 
strument can measure current and voltage as well as 
power. The measurement of these three quantities 
allows a calculation of the load impedance. Other 
features of the design are that the power range switch 
tends to keep the impedance range of the instrument 
constant and that the power measurement is inde- 
pendent of wave form. 

7 his meter is designed to operate for load impe- 




128 


USRL TEST STATIONS 



Figure 66. Circuit schematic of thermocouple wattmeter; 
circuit arrangements are shown for: (A) power measure- 
ment, (B) current measurement, (C) voltage measurement. 


dances from 10 to 300 ohms and has power scales of 
100, 500, and 1,000 watts. The indicated and actual 
scale factors agree within 1.5 per cent from 0 to 100 
kc. Operation outside the impedance limits will not 
only affect the accuracy and overload the thermo- 
couples but even may destroy them. 

While the thermocouple wattmeter is direct-read- 
ing and fairly accurate, the time required to reach 
temperature equilibrium delays the readings so that 
point by point measurements are required. The pos- 
sible destruction of the thermocouples from sudden 




I 


INPUT 




LOAD 


R, A 


O-c 


1 


R2 


Figure 67. Recording wattmeter circuit for unbalanced 
load. 


changes in load impedance and the limited impe- 
dance range for any one power scale are distinct dis- 
advantages. The recording wattmeter was designed 
to overcome these difficulties. 

Recordmg Wattmeter.^^^ In measurements with the 
recording wattmeter, two signals designated S and A 
are obtained and recorded. The signal is obtained 
by adding a signal which is n times the current i to 
one which is m times the voltage e. The signal A is the 
difference between ni and me. It can be shown that 

~ ^ - = ei cos 6 = Power, (2) 

4m?i 

where 0 is the phase angle between the current and 
voltage. The and A signals are recorded on the in- 
termediate-frequency systems in the usual manner. 
The measurement of total power is correct only for 
sinusoidal waves. For portable use, separate ampli- 
fiers and meters may be used in place of the recorders. 

Figures 67 and 68 show the connections used in 
obtaining the 2 and A terms for unbalanced and bal- 
anced circuits. 

In the actual circuit for an unbalanced load, a coil 
and pad replace the resistors in turn to obtain the 
2 or A signal. The voltage component is obtained by 
short-circuiting Ro and the current component, by 
disconnecting Ri. 

In the balanced condition, the only new factor to 
be considered is the stray capacity between the two 
high potential terminals of the driving coil. This may 
be neglected for the usual load impedances. The im- 
pedances of the secondary windings will not affect the 
measurements. 

It can also be shown that, if R 2 is center-tapped to 
ground, the meter will measure the power delivered 



Figure 68. Recording wattmeter circuit for balanced load. 




DESCRIPTION OF ORLANDO TEST STATION 


129 


to both the load impedance and the impedance to 
ground. However, if no gioimd connection is made, 
the meter will read the load dissipation independent 
of the degree of imbalance. 

The 2 or the A signal, depending upon the posi- 
tion of the reversing switch, will appear across R^, 
which consists of a pad pins coil. Voltage and current 
components are obtained in the same manner as in 
the unbalanced case by short-circuiting R 2 and dis- 
connecting 7^1, respectively. 

In the latest model of this wattmeter, the system 
can be changed from a balanced to an unbalanced 
condition by a switch on the front panel. In actual 
operations, four records are obtained on the chart 
paper, S, A, e, and i. Since the levels are recorded in 
db vs 10-16 ^vatt, the subtraction of a proper constant 
from the recorded X level will give in db vs 

1 watt. The same procedure and the constant are used 
to obtain A in the latter units. When these two read- 
ings are so converted, the difference between them 
will be the power delivered to the load. Similar pro- 
cedures are used to convert the voltage signal to db 
vs 1 volt and the current signal to db vs 1 ampere. 

It can be shown that the errors in the readings of 
the wattmeter are functions of the ratio rne/ni and 
three ranges of impedance have been incorporated to 
keep these errors at a minimum. If the records of cur- 
rent and voltage differ by more than some 8 db in a 
particular region, their product will be in error as 
the above ratio indicates. An impedance range should 
be selected which brings the two signals closer to- 
gether and thereby improves the accuracy of the 
power reading. These changes are readily made by 
means of a switch on the instrument. 

The only element affecting the frequency charac- 
teristic of the wattmeter is the loss on the insertion of 
the coil replacing R^. This will appear as a variation 
with frequency of the conversion constants but this 
effect over the range of 50 c to 150 kc is less than 0.4 
db. 

I'his wattmeter is used over a power range of 0.001 
to 1,500 watts and an impedance range of 15 to 800 
ohms. The accuracy is on the order of 1 to 4 per cent 
for phase angles up to 85 degrees. The accuracy be- 
yond this angle has not been completely investigated 
experimentally. 

Miscellaneous Equipment 

General laboratory apparatus is available at all 
times for use with the circuits that are being devel- 


oped and the construction that is in progress. This 
equipment includes decade condenser and resistance 
boxes, attenuators, storage batteries, and moderate 
stocks of fixed resistors, condensers, and inductors. 
Various switches and vacuum tidies are available in 
addition to transformers for power, signal frequen- 
cies, and variable voltage. 

A modified Hallicrafters short-wave receiving set 
is used as a voltmeter and a harmonic analyzer for 
low-level signals at the higher frequencies (15 me). It 
also provides an excellent means for detecting stray 
radiation. 

63 DESCRIPTION OF ORLANDO TEST 
STATION 

^ Site of Station 

The Orlando station of USRL is located on Lake 
Gem Mary about 4 miles southeast from the center of 
Orlando, Florida. The lake is almost circular with a 
diameter of some 300 yards, which is ample for cali- 
brations, yet it is not so large as to have high waves in 
windy weather. Typical of the lakes in this region, it 
is fed from subterranean sources and has no surface 
inlet or outlet streams. The depth of the lake varies 
with the amount of rainfall. This variation in lake 
level so seriously affects the calibration work that 
remedial measures are necessary. A pump is installed 
which draws water from an adjoining lake and auto- 
matically maintains the water level within ±0.25 
inches. The depths range from 15 to 18 feet under the 
test pier, which extends about 130 feet from shore, to 
33 feet at the barge location in the center. These 
depths are satisfactory for testing purposes. The lake 
bottom consists of sandy loam, except for the central 
deep portion of soft mud. The acoustic absorption 
of the loam, as determined by tests, is high and in- 
creases with frequency. The reflected sound is 10 db 
below the incident at 20 kc, 15 db at 30 kc, and 20 db 
at 60 kc. 

Facilities 

The Orlando station provides facilities for the free 
field calibration of underwater sound devices in the 
frequency range from about 15 c to 150 kc. One ob- 
ject in setting up this station was to have a place 
where tests could be made when the water at Moun- 
tain Lakes is frozen. For this reason, outdoor facilities 
were of most importance, since the indoor facilities of 


130 


USRL TEST STATIONS 



Figure 69. The Orlando test station as seen from the pier. Heavy equipment is loaded onto a mine car by means of 
the boom and chain hoist at the left. The rails on which the mine car runs are shown in the foreground. 


Mountain Lakes are usable all year round. A fun- 
damental requirement for the station was that the 
outdoor facilities in every essential respect be the 
ecjuivalent of those at Mountain Lakes. This has been 
attained. 

Orlando has two testing systems, one located on the 
pier, the other on a barge in the middle of the lake. 
I’he pier system is very similar to that at Mountain 
Lakes, but a number of modifications have been in- 
corporated in the barge system to take full advantage 
of the deeper water available. 

Pier System 

The test station proper is located on the eastern 
shore of the lake. As shown in Figure 69, a boom 
equipped with a chain hoist is provided at the load- 
ing platform. By the.se means, heavy equipment can 


be lifted out of a truck and loaded directly onto the 
mine car, which runs on a small spur track from the 
loading platform to the pier. At the point on the pier 
where the test pit starts, the equipment is transferred 
to a chain hoist running on an overhead rail to the 
outer end of the pier. Several chain hoists of dilTerent 
capacities are available. I'he pier is similar to the one 
at Mountain Lakes except for its length of 130 feet, 
which was required by the more gradual slope in 
order to reach a sufficient depth for testing. An awn- 
ing that can be moved along the pier was installed 
to protect personnel and ecpiipment from the heavy 
tropical rains. 

Transmitting and receiving booths are located on 
the pier. The electrical ecjuipment of each is equiva- 
lent to that in the corresjxjnding booths at Mountain 
Lakes. 




DESCRIPTION OF ORLANDO TEST STATION 


131 



Figure 70. General view of the Orlando testing pier. 



Figure 71. View of the Orlando pier from laboratory, 
rhe transmitting booth can l)e seen at the right. The 
receiving booth is at the far end. 


Figure 72. The receiving booth and the test basin at 
Orlando test station. A rotator designed for rotating a 
device around points other than the center of gravity can 
be seen at the far end of the basin. 





132 


USRL TEST STATIONS 



Figure 73. Sketch of offset rotator and synchro con- 
trolled rotator with attachment for taking directivity 
patterns of a transducer inside a dome. 


The types of carriages, suspension rods, and rotat- 
ors with synchro control are identical with those at 
Mountain Lakes. The carriage shown at the far end 
of the test basin in Figure 72 is especially designed 
for use in the rotation of devices around points other 
than the center of gravity. This carriage and an offset 
rotator proved so usefid that a synchro control was 
added. A further attachment was designed for taking 
vertical directivity patterns, such as that of an echo- 
sounding projector inside a dome. In this case, the 
dome is turned on its side, the projector mounted in- 
side at any desired bearing relative to the nose, and 
the two are then rotated together. 1 hese features are 
shown in Figures 73 and 74. 

File electric etjuipment for the pier station is 



Figure 74. \'iew of corrugated dome ou special rigging 
for directivity measurements. 


housed inside the main building with the panels ar- 
ranged in an arc facing the lake, as shown in Figure 
75. The operator is stationed between these panels 
and a desk for recording test data, and a window 
above gives a view of the pier. The electric apparatus 
is similar to that at Mountain Lakes with the excep- 
tion of the recorder and the high-power amplifier 
shown in Figure 78. Operation and circuit of the 
recorder have been described in Section 6.2.1. In 
this case, the drive by a double-armature motor is 
replaced by a magnetic clutch which makes contact 
between the rails of the pen carriage and either side 
of a continuously revolving disk. Coordination of the 
oscillator and the paper drive is obtained by driving 
both by one motor. The bay on the extreme left of 


DESCRIPTION OF ORLANDO TEST STATION 


133 



Figure 75. Arrangement of electrical equipment in the 
Orlando laboratory. 


Figure 75 is part of the power amplifier system de- 
scribed above. It is capable of delivering 1,500 watts 
of electric power at fretpiencies from 2 to 100 kc. 
Facilities are available for the interconnection with 
other parts of the electric system and for measuring 
and accurately controlling the power levels. A watt- 
meter of the type developed by USRL is used with 
this unit and a motor generator set furnishes the 440- 
volt 3-phase power required. 

Barge System 

A general view of the barge is shown in Figure 76. 
The floor and frame are supported by 88 barrels, 
which are grouped uniformly under the floor and sec- 
tionalized so that individual units can be removed 
for repair or replacement without seriously interfer- 
ing with the buoyancy of the barge. A pontoon is in- 
stalled on the side where the house containing the 
electric equipment is located to provide the addi- 
tional buoyancy necessitated by the uneven weight 
distribution. The barge is positioned by means of 
cables at the four corners attached to mushroom 
anchors imbedded in the lake bottom. Additional 
anchors of a screw type are connected by cables to 
winches which may be adjusted to level the barge 
and lower it in the water for greater stability. 

Equipment is transported from shore in a flat-bot- 
tom boat of 1-ton capacity and raised onto the barge 
with a swivel crane. 



Figure 76. of the Orlando barge. The barge is 

equipped with hoist arms at each end and a boom crane 
for handling equipment. 

The test basin is about 45 feet long and is fitted 
with steel rails which are spaced the same as at other 
USRL basins, to make possible the interchange of 
carriages between them. An overhead rail is used only 
in the center while wooden hoist arms, visible in Fig- 
ure 76, are provided at each end of the test area. The 
hoists are lighter than a full length rail and are better 
adapted to handle the long rods that are used on the 
barge to take full advantage of the greater water 
depth. If the surface and bottom are total reflectors, 
the optimum testing depth is one-half the total depth 
but, if the bottom is absor})tive, deeper testing would 
be advantageous. Actually, the tests are made at 
about one-half the water depth of 33 feet. Figure 77 
shows a line hydrophone being mounted on a rod for 
testing at this depth. Hydrophones can be suspended 
either horizontally or vertically and operated with 
a synchro-controlled rotator similar to the one on the 
pier. 

The building on the barge contains a complete 
electric testing system, including those facilities 
which are usually installed in the transmitting and 
receiving booths on the piers. The booths are not 
required here because of the short distance to the 
basin. The sending system is essentially the same as 
the one on the piers, but the detector is omitted from 
the receiving system so that the amplifier recorder is 
responsive to any signal within the frequency band 
of the system 200 c to 150 kc. This makes the system 



134 


IJSRL TEST STATIONS 



more susceptible to noise and interference, but sev- 
eral fixed filters are available to restrict the recei\ ing 
band when it is necessary to reduce the noise levels' 

The linear recorder is of the same design as the one 
on the pier. The polar recorder is synchro-controlled 
from the rotator. The barge is also etpiipped for pulse 
testing with circiuts which are identical with those 
at Mountain Lakes. The test-circuit cable from the 
pier is carried on the surface by drums spaced at 
various distances, while the power and communica- 
tion cables rest on the lake bottom. 

AVhen tests are to be made over greater distances 
than the barge or pier areas permit, the distance is 
extended by a small triangular float which can be 
anchored anywhere in the lake and which is equipped 
with a simple hoist. 


c This wide-hand system was used Ijecaiise it was available at 
the time the barge was built, whereas the coustructiou of a 
heterodyne and filter system would have entailed considerable 
delay. It may he noted that all the early testing systems of IJSRL 
\vere of the wide-hand type. 


Figure 77. Attaching a line hydrophone to special sus- 
pension rod on the Orlando barge. 


64 RECOMMENDATIONS FOR 

IMPROVEMENTS OF THE USRL 
TEST STATIONS 

The recommendations made in this section will 
deal mainly with improvements and additions that 
may be made to the existing electrical and mechani- 
cal components of the calibration systems. These 
improvements and additions are of an immediate 
practical nature and several are in the process of 
development. The general purpose is to increase the 
accuracy and ease with which the acoustic measure- 
ments are made and the resultant data converted 
into response, impedance, and other forms which 
characterize transducers. (See Chapter 4.) 

General Improvements— Electrical 

Under proper operating conditions the stability 
of the electric equipment now installed at USRL test 
stations is approximately 0.1 db. Signal generators 
are designed with ^'ery little drift in frequency. The 
gain of each individual component is as uniform as 
j30ssible over the frequency range for which it is de- 
signed. This is done so that the relation of gain to 


FuaiRE 78. Part of the electrical system on the Orlando 
barge, including the linear recorder. 



RECOMMENDATIONS FOR IMPROVEMENT 


135 



Figure 79. Small triangular float which can be anchored 
anywhere in the lake. Note the diver’s helmet which is 
useful for inspecting underwater etjuipment. 


frequency for each component may be neglected in 
the calculations. Although the present corrections lie 
within ±0.1 db, further improvement along these 
lines is possible. 

General Improvements— Mechanical 

Since the purpose of the mechanical systems in any 
acoustical calibration is to handle, hold, or orient 
the devices, any improvement which will reduce the 
time involved in rigging and measurement and still 
provide a minimum of interference in the sound field, 
is recommended. To minimize the number of systems 
required, each should be designed to handle as many 
different devices as possible. 

Electromechanical Equalizer 

Transducer calibrations would be greatly facili- 
tated if only there were projectors which give a coo- 


signal. 




INPUT 


EOUAUZER 

ATTENUATOR 

z: 



— AMPLIFIER — 


RECORDING 


RECORDER 

AND 

EQUALIZING 

SYSTEM 

SYNCHRONIZED 



EQUALIZING RECORDED 

CHART CHART 


Figure 80. Schematic diagram of electromechanical 
equalizer. 


stant sound field over the whole frequency range, or 
hydrophones which had a uniform response for a 
constant field at all frequencies, or both. Either in- 
strument would make possible the direct calibration 
of the other. Though no such ideal instruments are 
available, it is possible to approach the same result by 
proper control of the amplifier gain, or, what is more 
convenient, by control of the attenuation preceding 
a constant amplifier gain. The method is indicated 
in Figure 80. 

Hydrophones are available which have been cali- 
brated by reciprocity or other means. Using such an 
instrument with a fixed attenuation and amplifier 
gain, a recording is made as the projector sweeps over 
the range of frequencies. From the calibration curve 
of the hydrophone, a curve may be plotted which will 
give the strength of the sound field produced by the 
projector at each frequency. If a straight line is drawn 
parallel to the recorder axis and at some desired 
sound level, the difference between it and the projec- 
tor curve will be the numl^er of decibels by which 
the actual field differs from a constant one. If some 
device connected to the attenuator can be made to 
follow the curve and thus \ ary the out})ut level so that 
it will be proportional to the de\ iation of the curve 
from a straight line, the output of the amplifier will 
be at a constant level. In other words, the record 
would be that of a flat hydrophone in a constant 
sound field. If now' the coupling loss of an unknowai 
hydrophone is determined and the corrections for 
this loss added to the original curve, the resulting 
control curve wall cause the attenuator to correct for 
both the variations in the projector output and the 
coupling loss. If this hydrophone is placed in the 
field of the projector and its output is fed through 
the attenuator w'ith its gain controlled by the curve, 
the hydrophone signal w'ill have added, at every fre- 
quency, the number of db necessary to make its out- 
put what it w'ould have been if the projector output 
had been constant. In other words, the record is the 
response of the unknow'n hydrophone in a constant 
sound field. The same technique wdll give a correc- 
tion curve for a hydrophone and allow the direct 
calibration of a projector. Calibration by this meth- 
od eliminates the time and errors involved in the 
point by point computations required at present. 
T he data are in such a form that the calibration can 
be reproduced directly by photographing the rec- 
orded chart. 

Such a system is under consideration by USRL, 


136 


USRL TEST STATIONS 


and a tentative design is partially completed. The 
curve tracer mechanism is based on a light-beam and 
photoelectric cell null-balancing scheme. 

Transient Wave Analyzer 

In the study of wave forms, the use of the Henrici 
analyzer for the measurement and analysis of tran- 
sients has proved both time-consuming and expen- 
sive. Therefore, USRL later developed an instrument 
which not only may be constructed from inexpensive 
and easily available parts but also speeds up the proc- 
ess of analysis. (See Figure 81.) This transparent 
cylinder is rotated by a motor at about 1,800 rpm 
running between a light source on the inside and a 
narrow slit outside. The light passing through the 
slit impinges on a photoelectric cell of the vacuum 
photomultiplier type. The associated tube circuit for 
amplification is shown at the right of the figure and 
the power supply at the left. The usual recording sys- 
tem for any steady-state signal such as system 2 is 
used beyond this point. 

In operation, an oscillogram of the transient is re- 
produced as an opaque stencil and attached to the 
surface of the cylinder. It is then rotated between the 
light source and the photoelectric cell producing an 
electric signal corresponding to the transient which 
is repeated some thirty times each second. The en- 
velope of the amplitudes of the Fourier components 
of this signal is proportional to the spectral distribu- 
tion of energy in the transient. This envelope may be 
obtained directly as a function of the frequency by 
using system 2 with the 300-c band which will average 
several adjacent harmonics. The record obtained will 
be independent of the lowest Fourier frequency as 
long as it is small compared to 300 c, since changing 
the frequency and amplitude of the linear sweep cor- 
responds to changing the scale factor in a Henrici 
analysis. (See references 58 and 79.) Calculations 
made from this record are identical with those from 
the Henrici analyzer. 

After a careful adjustment to eliminate distortion, 
several transient sounds from Navy devices were ana- 
lyzed by this method. The results were carefully 
checked to determine their validity. The accuracy of 
the reproduction was tested by viewing it on the 
CRO, and the broad-band rms level of the original 
transient was compared with the one delivered by 
this apparatus. As an additional check, a square wave 
was analyzed. Since the Fourier analysis of this wave 
is mathematically known, the analysis of the instru- 



Figure 81. Optical signal generator of apparatus for 
transient analysis. 


ment could be readily compared with the theoretical 
values. Good agreement was found to exist between 
the two. 

Thpre are minor improvements which could be 
made, but the instrument as it stands is workable 
and has adequate precision. The speed and facility 
in the analysis of pulse signals have been very much 
improved. Obviously, the method may be applied to 
electrical pulses from any source. 

If no analyzer such as system 2 is available, the am- 
plitudes of the Fourier components may be deter- 
mined with a commercial electric harmonic analyzer. 
The phases of the components, however, cannot be 
determined by either method. 

Acoustic Phase Measurements 

For a more complete characterization of transduc- 
ers, an instrument is desired which measures the 
phase between the acoustic signal and the electric 
signal. This would be of advantage, particularly in 
the analysis of transient wave forms. 

Phase bridges are available^’®- that measure the 
relative phase between two electric signals. This 
limits measurements in acoustic tests to the difference 
in phase between the current into a projector and the 
voltage generated by the hydrophone. However, the 
reciprocity relation of a transducer indicates that the 
phase shift between the current and the generated 
pressure when acting as a projector, minus the phase 
shift between the applied pressure and the open-cir- 
cuit voltage when acting as a hydrophone, is either 
180 or 0 degrees for a magnetostrictive or a piezoelec- 


RECOMMENDATIONS FOR IMPROVEMENT 


137 


trie instriinient. This assumption or preferably an 
exact knowledge of the phase constant plus the phase 
measurements on the three possible combinations of 
a projector, hydrophone, and reversible transducer, 
would be sufficient to obtain the phase shift between 
the incident pressure of the hydrophone and its gen- 
erated voltage. 1 he time delay in the acoustic medi- 
um would have to be taken into account, but once 
such a standard had been calibrated, an unknown 
could be determined by comparison. Measurements 
of this nature have not been made to date by the 
Underwater Sound Reference Laboratories. 

Improved Variable Band-Pass Filter 

An instrument which proves of value in both the 
analysis of noise and the characterization of transduc- 
ers is a continuously variable width band-pass filter 
centered at any given frequency. I'he filter of this 
nature described previously requires too much auxil- 
iary equipment and is awkward to use. A simpler and 
self-contained one is most desirable. For example, it 
is often important to describe a hydrophone’s per- 
formance in terms of the response to a given band 
width of noise centered at a specific frequency. Such 
a filter could be readily used in conjunction with a 
noise source to provide the necessary signal. 

Improved Pulse Recorder 

One of the inconveniences of the present pulse 
recorder is the fact that its sensitivity and perform- 
ance are not independent of the pulse length and 
rate of repetition. A recorder to overcome these diffi- 
culties is very desirable. 

Recording Impedance Bridge 

A continuous-recording impedance bridge would 
be of great value to any laboratory concerned with 
electric and acoustic measurements. The recording 


wattmeter already described can be used for imped- 
ance measurements and with high accuracy if com- 
bined with an insertion-type phase bridge mentioned 
above. Both the ratio of voltage to current and the 
phase could be determined by the amount of attenua- 
tion called for by the two null-balancing circuits. 
Recorders of the same general type as those used in 
the electric systems of USRL laboratories would pro- 
vide a continuous record of the attenuation inserted 
and hence a continuous record of the phase and mag- 
nitude of the impedance. 

Absorbing Materials 

A perfect absorbing material would provide almost 
ideal testing conditions when operating in small 
space. It is felt that there is a possibility of producing 
better absorbing materials or combinations of mate- 
rials than have been developed up to the present 
time. Any improvement along these lines would be 
of aid not only in calibration techniques but also in 
the design and application of acoustic devices. 

Directivity Index Measurement 

The directivity index of a projector is necessary 
for the computation of its efficiency. In general, this 
index can be obtained only by computation from the 
directivity patterns and even then with limited ac- 
curacy. Because of the large amount of time required 
for these computations, the University of California 
Division of War Research has built a system for mak- 
ing direct measurements of the directivity index. It 
consists in rigging the projector so that it can be 
rotated and the acoustic output integrated over the 
surface of a surrounding sphere by means of the watt- 
hour meter. The directivity index is the ratio in db 
of this integrated power to the area of the sphere 
times the acoustic power per unit area on the acoustic 
axis. 


Chapter 7 

COMPUTATION FROM TEST DATA 

By Eginhard Dietze and L. Pauline Leighton 


T he method of computing the calibration of an 
instrument from the test data is described in this 
chapter. Reference should be made to Chapter 6, 
which outlines the procedure for taking test data, 
and to Chapter 4, which gives the theoretical back- 
ground for these computations. 

For a numerical example, the calibration of a trans- 
ducer is given, since such a unit can be operated as 
both a receiving and a sending device, affording the 
opportunity of illustrating both types of computation. 

7 1 RECEIVING RESPONSE 

To test the receiving response over a wide frequency 
range, several sets of tests with a number of projectors 
may have to be made. However, since the computa- 
tion is the same at all frequencies, only one set of data 
is discussed here, and the numerical computation is 
limited to one frequency, 25 kc. 

It is assumed that calibrated standards are available 
so that the comparison method may be used. This is 
the usual test procedure as discussed in Chapter 6. 
The computation of the calibration of the standards 
themselves by reciprocity is discussed in Section 7.4. 

Usually several standards are used in one test to 
provide a mutual check on their performance. In this 
discussion, however, attention is confined to a single 
standard, a type 3A hydrophone. The calibration of 


(/) ^ 
ife 





Figure 1. Calibration of 3A89 crystal hydrophone. 


the 3A standard, serial No. 89, used in this particular 
illustration is shown in Figure 1. 

The data furnished by the test station are illus- 
trated by the receiving response charts. Figures 2 and 
3, and by Figure 4, which shows a log sheet applicable 
to these tests. Figure 2 shows a receiving chart for the 
3A89 hydrophone and Figure 3 a receiving chart for 
the transducer under test. The conditions of the tests 
applicable to these two receiving charts are stated on 
the log sheet, from which can be obtained: 

1. The testing distance between the source and the 
hydrophone. 

2. The available power that was used for the pro- 
jector. 

3. The receiving amplifier gain that was used in 
the tests. 

4. A reference to the circuit sketch for this test. 

With these data on hand, the calibration can pro- 
ceed. The first item to be checked is whether or not 
the same testing distance was employed for the test 
unit as for the hydrophone standard. If not, the read- 
ings must be corrected to the same testing distance by 
means of the formula given in Chapter 4, 

C = 20 log 4 (1) 

where d and do are the testing distances for the test 
unit and for the hydrophone standard respectively. 

It will be recalled that the receiving response usu- 
ally is expressed in terms of the generated voltage of a 
hydrophone. One exception to this rule is in the cali- 
bration of the 3A hydrophones, in which the response 
is expressed in terms of the voltage across 135 ohms. 

The receiving charts give the level (in db vs 10~i® 
watt) impressed on the recorder. A number of correc- 
tions have to be applied to obtain the generated volt- 
age of the hydrophone from the chart readings. Cor- 
rections must be made for (1) the receiving amplifier 
gain, and (2) the coupling loss. By the latter is meant 
the loss (or gain) in the voltage applied to the input to 
the receiving amplifier as compared to the generated 
voltage of the hydrophone. Since the loss in the line 


138 


RECEIVING RESPONSE 


139 



Figure 2. Receiving response chart, 3A89 hydrophone vs AX70 projector. Reference run No. 1 on log sheet (Figure 4). 


from the pier to the receiving amplifier is included in 
the calibration of the system, the voltage at the line 
terminals may be substituted for the voltage at the 
input to the receiving amplifier. 

Referring to the example, the log sheet in Figure 4 
contains a reference to the circuits used for the stand- 


ard and the transducer under test. These circuits are 
reproduced in Figures 5 and 6. It may be seen that the 
transducer was connected through a balanced coup- 
ling amplifier to the 135-ohm receiving line. To eval- 
uate the coupling loss of this amplifier, two measure- 
ments were made, as shown in Figure 7. First, a volt- 



Figure 3. Receiving respon.se chart, QB No. Ill vs AX70 projector. Reference run No. 2 on log sheet (Figure 4). 


140 


COMPUTATION FROM TEST DATA 


LOG 


PROJECT t^O. OOO DATE 


_TEST STATION / SYSTFM; 

-COPIES TO STATION FILE AND N. Y OFFICE -2 


-SHEET NO. H_ 


INDEX 

WATER 

TEMP. 

TRANSMITTING 

FREQUENCY-KC 

RATES 

RECEIVING 

Run 

No. 


TIME 

PROJ. 

TYPE-NO. 


PIER 

POS. 

CM 

Depth 

CM. 

AVAIL. 

POWER 

SINGLE. 
RANGE, or 
MID-BAND 

Band W th PULSE 


HYDRO 

TYPE-NO. 


Depth 

CM 

Horii 

Disc 

CM 

Sys 

Gtia 

db 

Type 

Color 

Let- 

REF. 

CCT. 

Deg. 

C 

Depth 

CM 

Tf. 

Rec. 

Lgth-MS 

Dly 

MS 

R»te 

PPS 

s«-p 

P*pf 

IPM 

Ron 

RPM 

Zo 

Level 

Te. 

Rec. 

/ 

R 

BL 

A 

I 

9:30 

20 

Z$0 

AX 70 


too 

2(.% 



2. 5-/00 







Tn 

6 


3A 89 

^?o' 

2a 

250 

50 

2 

R 

BL 

B 

H 

9:9-0 

" 


AX70 


// 

ft 


" 

5-9o 







07) 

(p 




" 

tf 

59 


CO 

G 

C 

m 

950 





» 0 




o-no 







/ 

/S 






20 

fX 

d 

R 

C 

ur 

9:50 



yO 



Zn 

• 


II 







J 

/S 






20 








5 

R 

BL 

D 

IT 

10:50 


2^0 

QB^in 


lOO 

2i,i 

/35 


IO-C,o 







nf) 

6 


3A^9 

i-9& 

'2a 

250 

9 

6X 

R 

R 

E 

:sL 

YTL 

10:90 

/0:9-S 

\Ai 





— 

— 

— 

0'/5O 








/S 






0 

/Oc/^ 





7X 

R 

G 

E 

■Cu'nA. 





/3S^ 

*/5o 

0-/50 







J 

/^ 






0 
















/ 












T 















































































1 

1 























































































































































































































































rLOG(10 

-4-4) ‘R-RESPONSE; CI-C 

PL'G IN; CC 

ic'PnroDT 


mAK 

• IN-INH N 

T NOlSt; Na-NO 

SE ACTi 

\TN 

i-pRoj a 

jR; fe-rn 

)J V 

LT A & 2 PROJ PR 


Figure 4. Sample log sheet covering calibration of QB No. Ill transducer. 


age was applied to the receiving line from a very low 
impedance, 0.034 ohm, as shown in (a) of Figure 7; 
this same voltage was then applied to the receiving 



ELEVATION 

Figure 5. Circuit used in obtaining reference receiving 
response chart, 3A89 hydrophone. 



Figure 6. Circuit used in obtaining receiving response 
chart, QB No. Ill transducer. 


amplifier through the coupling amplifier, as shown 
in (b) of Figure 7. The two recorder traces are shown 
in Figure 8. The log sheet shows that both measure- 
ments were made with the same receiving amplifier 
gain. Thus, the difference between the two traces 
gives the coupling gain or loss. It will be seen that 
this loss is 6.2 db at 25 kc. 

The generated voltage of the test instrument rela- 
tive to the voltage of the 3 A hydrophone across 135 
ohms can now be computed as follows: 


Difference in receiving chart readings -f 15.2 db 

(From Figures 2 and 3) 

Coupling loss for test hydrophone -f 6.2 db 

(From Figure 8) 

Difference in receiving amplifier gain —4.0 db 

(From Figure 4) 

Difference in available power to projector ... 0 db 

(From Figure 4) 

Difference in testing distance 0 db 

(From Figure 4) 

+ 17.4db 


With the calibration of the hydrophone standard 
(Figure 1) known, the receiving response of the test 
instrument may be obtained as follows: 

The response of the standard, according to Figure 
1, is —98.2 db vs 1 volt for a sound field of 1 dyne per 
sq cm at 25 kc. The response of the test instrument, in 
accordance with the above computation, is 17.4 db 


THRESHOLD AND IMPEDANCE 


141 


131 


0 . 034 ^ 


>“ 



\ — 



n 



REC LINE 
*= 135 '*’ 


A 


REC LINE 
*= 135 «-* 


B 

Figure 7. Circuits used in obtaining coupling loss for 
3A89 hydrophone. (A) For measuring input voltage (see 
run No. 4, Figure 4). (B) For measuring output voltage 
(see run No. 3, Figure 4). 

higher. Thus the result obtained is that the projector 
at 25 kc has a receiving response of —80.8 db vs 1 volt 
per dyne per sq cm. 

The preceding computation, which was carried 
through at one frequency only, is repeated at selected 
frequencies throughout the entire frequency range. 
From these data, a response characteristic is plotted as 
shown in Figure 9. 

In these particular tests a 3A hydrophone, which is 
a pressure-actuated device, was employed as a stand- 
ard. If the hydrophone standard is of the pressure- 
gradient type, such as the lA or 2 A hydrophone, it 
generates a gain in the voltage, especially at low fre- 
quencies, due to the curvature of the wave front. Fig- 


i35'-:ao34“ 


*il35* ^ ^ ^ ^ 


BALANCED 

COUPLING 

AMPLIFIER 


ure 13 in Chapter 5 shows the magnitude of the spher- 
ical wave correction for different testing distances, 
plotted against frequency. The indicated response of 
the test hydrophone, because of this gain in the hydro- 
phone standard (if the latter is of the pressure-gradient 
type), is lower than it would be if the tests were made 
in a plane-wave sound field. To refer to plane-wave 
conditions, therefore, the relative response must be 
correspondingly increased. 

72 THRESHOLD AND IMPEDANCE 

After the receiving response characteristic has been 
determined, it is possible to compute the threshold 
characteristic. Equation (18) in Chapter 4 gives an ex- 
pression for the threshold pressure. It can be seen that 
this pressure depends on the resistance of the hydro- 
phone as well as on its response. It is therefore neces- 
sary to compute this resistance. The chart in Figure 
10 shows readings for the projector taken at the test 
station by means of the 5A impedance bridge. This 
bridge gives the admittance in terms of parallel resist- 
ance and capacity values. Since the bridge can meas- 
ure directly only impedances below 1,000 ohms, it is 
necessary to shunt the unknown impedance whenever 
it exceeds that value. The bridge shunt resistance used 
is given in Figure 10 as and the bridge resistance 
reading as Ri,. 






























































































^IN 

PUT 




























































JTPUT 
















































1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 

KILOCYCLES PER SECOND 


Figure 8. Chart of coupling loss for 3A89 hydrophone. 


OPEN CIRCUIT VOLTS IN DB VS I VOLT 


142 


COMPUTATION FROM TEST DATA 


-70- 


o -80- 


Q -90- 

- 

g -K)o- 
y 

u. 

9 -no- 


< -t20- 
(T 

e 

-130, 


OJ 



1 10 
FREQUENCY KC 



Figure 9. Receiving response of QB No. 1 1 1 transducer. 


The first step in computing the impedance from 
the values on the chart is to compute the unknown 
parallel resistance, which is 


From the computed shunt resistance R and the 
shunt capacity C (given in microfarads on the chart), 
the equivalent series resistance r and series reactance 
.Y are then computed by means of the formulas 


and 


R 

1 -h (wRcy 


<oR-C 

1 + (ioRcy^' 


( 3 ) 

(“I) 


This process, carried through numerically at 25 kc, 
gives 


R = 


1,007.7 X 781.5 
226.2 

3,450 

’ 1 + 394 


= 3,450 ohms, 

8.7 ohms. 


R = 


RpRb 

Rq ~ 


( 2 ) 


and 


68,200 

1 -h 394 


— 173 ohms. 


IMPEOAMCE 

a.Test Station 


Project Ho. 9 

Perice; Desi?. QB^/H 
Measaria; cosditioas: 

Local iOB of device: 
CrtMsdiiig effects 
Bridfe asd laiiliaries: 

Osc. 17 B 



__ . ■ 

^jC. a leakage Besistaace: 


Brid{e_£A_ 


lead to lead 
L-t- L- 


'3al. %/ Tabal. 
Detector 3/ ATh^S 


R±. 



g-Q V >30^7«^Sfcieldsto Case 

\ Carre^Jt tkroa^k Pridee O' 

-■ Heasared ralie of Skaatiag Besistaace 
= Valae of Prid^e Besistaace balanced 
aeaiast Besistaace of Uakaowa. 
note: Ckeck ia first colana aader B^ ladi- 
cates aakaovB kas beea skaated by B^- 
-CJ* = Talae of Bridge Capacitaace balaaced 
agaisst Capacitance of Caknowa. 

•O Talae of Bridge Capacitance to Besoa- 
aie witk Indactaace of Uakaona. 


lead to lead ofciis 

L- >3o. 

okield 

fSkieldsio Case >_^ 




767 2 KUOSi, 


10069 ✓ 


9603-037201 


l0O73\y/ 


9373-039226 


J007.S y 


g22.2-Of/l63 


. 22 . 


10097 y 


7i/.Sk 07/525 


I007.9W 


736.%V.O9iS70 


10D7.7\J 


2UL 


r 07/525 


J007.SW 


697,0 


-.092150 


_28. 


10079 y 


655.gp072y3< 


30 


/0O7<f\V ' bfg 0-092651 


59/3 V.092M 

560-6^.09rW 


'oogo\y 


'^oog.2 2 56O-6L.O»f7O0 

\oog.^ J0S7.7\^.O5O5S^ 


SO 


/QOS S' 


599.3 r.O6O930 




l/009 2> 


2997- 073952 




J0092 


692 -.115965 


J Li- 


This computation is repieated at other frequencies, 
and an impedance characteristic is plotted for the in- 
strument, as shown in Figure 11. The threshold then 
is computed by means of equation (18) in Chapter 4. 
This computation at 25 kc is as follows: 

T = 10 log r - 194.9 - Rr 
= 10 log (8.7) - 194.9 T 80.8 
= — 104.7 db vs 1 dyne per sq cm. 



Figure 10. Sample data sheet giving 5-\ impedance bridge Figure 11. Impedance characteristic of QB No. Ill 

readings obtained in measuring the impiedance of the QB transducer. 

No. 1 1 1 transducer. 


TRANSMITTING RESPONSE 


143 



Figure 12. Transmitting chart, QB No. Ill transducer \’s 3.A89 hydrophone (see run No. 5, Figure 4). 


7^ TRANSMITTING RESPONSE 

Transmitting tests are similar to the receiving tests 
described above. Figure 12 shows the transmitting 
chart of the projector, using for the sound receiver 
the same 3A hydrophone standard as in the receiving 
tests. Reference should be made again to the log sheet 
(Figure 4) for the details of this run. 

This log sheet shows that the available power sup- 
plied to the projector was 160 db vs 10“^® watt (i.e., 1 
available watt) from 135 ohms. The log sheet also re- 
fers to the test circuit used in the transmitting tests. 
This circuit is reproduced in Figure 13. The receiving 
amplifier gain given on the log sheet is 4 db, and the 
testing distance is given as 250 cm. 

From this information it is {X)ssible to compute the 
transmitting response. This computation is carried 


through numerically at 25 kc as follows: 

Chart reading, vs 10— 16 watt in 135-ohm circuit . -t-118db 

Correction for receiving amplifier gain — 4 db 

Correction for testing distance: 20 log 2.5/ 1 . -t-8db 

Correction for available power referred to 1 available 

watt 0 db 

Level at hydrophone terminals, >’s 10—16 watt in 135- 
ohm circuit -l-122db 


In order to obtain the pressure in the sound field, it 
is now necessary to refer to the calibration of the 3A 
hydrophone shown in Figure 1. This calibration is in 


terms of db vs 1 volt. The computed level at the hydro- 
phone must, therefore, be changed into these terms. 
This correction is as follow’s: 

W'^hen the power dissipated in a 135-ohm resistance 
is 10~^® watt, the voltage e across that resistance is 
given by the relation 


so that 


_£L 

135 


10-16 


= 135 X 10-16. 


On a decibel basis relative to 1 volt, this gives 
20 log e = 101og(135) - 160 
= —138.7 db vs 1 volt. 



POWER •teo* vs lo"'* WATT 


BATTW SUPPLY 
FOR 

3A MYBROPMONE 


I 


REC LMC 


4 * 'V* SCREEN 



elevation 


Figure 13. Circuit used in obtaining transmitting chart, 
QB No. 1 1 1 transducer. 


144 


COMPUTATION FROM TEST DATA 






























































































'REC 

SYST 

EM 


















N 





















TRAN 

IS+RE 

:C S'! 

'STEN 

1 












































I 2 3 4 5 6 7 8 9 10 20 30 40 50 60 

KILOCYCLES PER SECOND 


Figure 14. Response characteristics of transmitting and receiving systems during calibrations. 


The voltage delivered by the 3A hydrophone conse- 
quently is 

122.0 - 138.7 = -16.7 db vs 1 volt. 

The calibration chart (Figure 1) shows that, if the 
sound pressure is 1 dyne per sq cm, the hydrophone 
delivers —98.2 db vs 1 volt. For the voltage to be 
— 16.7 db, the sound pressure must have been 81.5 db 
above 1 dyne per sq cm. Thus, the transmitting re- 
sponse of the projector at 25 kc is found to be +81.5 
db vs 1 dyne per sq cm. 



Figure 15. Transmitting response of QB No. Ill 
transducer. 


The electrical system is usually adjusted at one fre- 
quency, and the sensitivity may vary somewhat with 
frequency. When this is the case, a compensating cor- 
rection must be made in the transmitting response. 
Figure 14 shows response characteristics of the trans- 
mitting and receiving systems used in the present 
tests. At 25 kc this correction amounts to about 0.5 db. 
The transmitting response must therefore be in- 
creased by this amount, i.e., the transmitting response 
is actually 82.0 db vs 1 dyne per sq cm. 

This computation when carried out over the fre- 
quency range gives the transmitting response charac- 
teristic of the projector, shown in Figure 15. 

After the transmitting response has been obtained, 
the projector efficiency is computed. This requires a 
knowledge of the directivity at the frequency at which 
the computation is made. Figure 16 shows a directiv- 
ity pattern of the projector taken at 25 kc. Chapter 4 
contains descriptions of graphical charts which facili- 
tate the computation of the directivity index from the 
measured directivity pattern. Since the side lobes of 
this particular pattern are low, it is possible to base 
the computation on the relation between the beam 
width and the directivity index for a circular piston 
(see Figure 7 in Chapter 4). The beam width in this 
case is 19.8 degrees, giving a directivity index of —24.2 
db by the chart. 

Equation (8) in Chapter 4 gives the expression for 


RECIPROCITY CALIBRATION OF STANDARDS 


145 



90 - 


Figure 16. Measured directivity pattern of QB No. Ill 
transducer at 25 kc. Directivity index computed from this 
pattern = — 24.1 dh. 


the projector efficiency. The only factor still unknown 
in this expression is 10 log PjIPa- This expression can 
be evaluated from the impedance of the projector and 
the source impedance (see ecpiation (2) in Chapter 4); 


'01og^= 10Iog[(,^+%‘'-^] 


= 10 


, r4x 8.7x1351 

L i43.7^+ Is TI 


= -9.6 db. 


The efficiency can now be stated 
£p= +82.0-24.2 + 9.6- 70.9 = -3.5 db vs ideal. 



r 2f/\ 

/ = 201og|^— X 10-’ 


74 RECIPROCITY CALIBRATION 
OF STANDARDS 

1 he comparison method of'calibrating acoustic de- 
vices depends on the availability of standards whose 
calibration is accurately known. The next problem, 
then, is to calibrate these standards. As discussed in 
Chapter 5, the best method for obtaining an absolute 
calibration is by means of the reciprocity principle, 
which permits a determination of the response from 
purely electrical measurements. 1 he reciprocity cali- 
bration requires considerably more work and is more 
critical than a relative calibration. Therefore its use 
should be confined to the fundamental calibration of 
hydrophone standards, with which all other instru- 
ments can then be compared. 

The relation by means of which the reciprocity cali- 
bration of a hydrophone can be obtained is given in 
Chapter 5. In decibel form this equation is 

= V 2 U + 20 log ej, + 20 log Cf,' 

-201ogc, -201ogi] (5) 

where / = 20 log {2dXl pc x 10“7). The term J is the 
reciprocity constant. This parameter for water for a 
number of testing distances is shown in Figure 17. 
The term is the open-circuit voltage generated by 
the device in a given sound field, is the open-circuit 
voltage generated by an auxiliary transducer in the 
same sound field, and is the open-circuit voltage of 
the device in the sound field produced by the auxil- 
iary transducer when a current i flows through it. 

The data usually are furnished by the test station 
in terms of level in db vs 10“4® watt rather than volt- 
age. The matter of translating levels into db vs 1 volt 
is discussed in Section 7.3. Since most of the circuits 
used at the test stations have an impedance of 135 
ohms, it is sufficient here to consider that case, in 
which the correction is —138.7 db. The above equa- 
tion can now be written in terms of levels: 

Pf — V 2 U “I" Ph + Ph ~ ~ 20 log i — 138.7]. (6) 

Usually the same transducer is used for the calibra- 
tion of a number of hydrophones. Then the following 
quantities, which are independent of the particular 
hydrophone being calibrated, can be computed, and 
the result used as a constant k in the other reciprocity 
computations: 


k = /-L^-201ogf- 138.7. 


( 7 ) 


146 


COMPUTATION PROM PTEST DATA 




KILOCYCLES PER SECOND 


Figure 18. Receiving chart. 1K13 projector acting as a Figure 19. Chart of current in the 1K25 projector acting 

hydrophone vs 1K25 projector acting as a sound source. as a sound source. 


In the following, the computation of the absolute 
reciprocity calibration of a 3A hydrophone is carried 
out at 1 ,000 c from test data which have been obtained 
at the station. In these tests, the 1K13 projector was 
used as the auxiliary reversible transducer and the 
1K25 as the other sound source. 

Figure 18 shows a receiving chart for the 1K13 pro- 
jector, taken with the 1K25 projector acting as the 
sound source. 

The pertinent test conditions are recorded as usual 
on a log sheet. The following information at 1,000 c 
is taken from that sheet: 

Transmitting conditions for 1K25 projector 
Source impedance = 4 ohms 
Available power =150 db vs 10 — 16 watt 

Testing distance = 45.7 cm 

Receiving conditions for 1K13 projector 
Coupling loss = 0.6 db (as shown by chart readings— 
see Section 7.1) 

Receiving circuit = 135 ohms 
Receiving amplifier gain = 30 db 


Figure 19 shows a chart of the current in the 1K25 
projector when 150 db vs 10“^® watt available power 
is supplied to it from a 4-ohm source. In order to con- 
vert the current chart reading into db vs 1 ampere, it 
is necessary to use the current-measuring circuit cali- 
bration. In this particular case, this calibration is as 
follows: The level indicated by the recorder when 1 
ampere flows in the current-measuring circuit is 112 


db vs watt. Thus 112 db must be subtracted 

from the chart readings. 

In addition to the chart in Figure 17 giving the 
value of the reciprocity constant J, the charts in Fig- 
ures 18 and 19 are sufficient for the computation of 
the constant k. 

It is desirable in this computation to reduce all 
values to a standard test condition which will be used 
in the subsequent tests on the hydrophones whose ab- 
solute calibrations are to be obtained. This test condi- 
tion will be based on (1) an available power level for 
the projector of 160 db vs lO-i® watt, and (2) a testing 
distance of 30.5 cm. 

The value for the reciprocity constant / can be read 
from the chart in Figure 17. At 1,000 c for a testing 
distance of 30.5 cm this figure is — 164.3 db. 

The level received in the 135-ohm circuit is ob- 
tained as follows: 

Receiving cbart reading (Figure 19) + 107.6 db vs 10 — 16 watt 

Correction for receiving amplifier gain —30.0 
Correction for coupling loss +0.6 


Level received in the 135-ohm circuit 78.2 db vs 10 — 16 watt 


This level must be reduced to standard conditions by 
two corrections: 

1 . The testing distance was 45.7 cm. If the measure- 


RECIPROCITY CALIBRATION OF STANDARDS 


147 
















1^ 




• tk 


jA'yn 

' 





1 

f 


















1 










1 2 3456789 10 20 

KILOCYCLES PER SECOND 



KILOCYCLES PER SECOND 


Figure 20. Receiving chart. 3A74 hydrophone vs 1K13 Figure 21. Receiving chart. 3.\74 hydrophone vs 1K25 

projector as the sound source. projector as the sound source. 


ments had been made at 30.5 cm, the level would 
have been increased by 

2. The available power into the projector was 150 
db vs 10~i® watt. ^Vith 160 db available power, 
the level would be increased by +10.0 db. 

Making these two corrections gives 

1 . 1 = 91.7 db vs 10“i‘'’ watt. 

The current chart reading in Figure 19 at 1,000 c is 
127 db vs 10~i® watt. From this reading must be sub- 
tracted the receiving amplifier gain, in this case 40 db, 
and the correction factor which converts the reading 
into db vs 1 ampere, 1 12 db, as stated above. In addi- 
tion, the reading must be increased by 10 db, on the 
basis that the standard test condition uses an avail- 
able power level of 160 db rather than 150 db. Thus, 

20 log / = (127 - 40 - 112 + 10) 

= — 15 db vs 1 ampere. 


Thus the constant k is obtained: 

k = -164.3 - 91.7 + 15 - 138.7 = -379.7 db. 

The reciprocity calibration of any desired hydro- 
phone can now be obtained from two receiving level 
charts, and L;,', taken of that instrument with the 
1K13 projector and with the 1K25 projector. These 
data must be for the test condition for which k was 
computed, that is, a testing distance of 30.5 cm, and 
an available power into the projector of 160 db vs 
10-1® watt, or else corrections must be made to reduce 
the data to these conditions. 

Figure 20 shows a receiving chart for the 3A74 hy- 
drophone taken with the 1 K13 projector as the source. 
Figure 21 shows a receiving chart for the same instru- 
ment taken with the 1K25 projector as the source. 
Both charts were taken at a testing distance of 30.5 
cm, with an available power of 160 db applied to the 
projector. Consequently, no corrections for testing 
distance or available power need be made. A correc- 
tion, however, must be made for the receiving ampli- 
fier gain, which was 20 db in both cases. Since the 
hydrophone in both tests was across a 600-ohm line, 
no coupling correction is required if the receiving re- 
sponse is desired in terms of the output ^'oltage from 
the preamplifier across 600 ohms. 

Reference to the chart in Figure 20 shows the level 


148 


COMPUTATION FROM TEST DATA 


reading at 1,000 c to be 114.8 db vs watt. Cor- 

recting this for the receiving gain (20 db) gives 

L,^ = 94.8 db vs lO-^*^ watt. 

Similarly, from the chart in Figure 21, 

L;/ = 115 - 20 = 95dbvs lO-i^^att. 


Thus, the absolute calibration of the 3A hydrophone 
can be computed at 1,000 c: 

Rr = 1/2 (-379.7 + 94.8 + 95) 

= —95.0 db vs 1 volt across 600 ohms 

for a sound field of 1 dyne per sq cm. 


Chapter 8 


PRODUCTION TESTING OF SONAR TRANSDUCERS 

By Erwin F. Shrader \ 


8 1 GENERAL CONSIDERATIONS 

P RODUCTION testing may be distinguished from type 
testing. Before a design is adopted, very careful 
calibrations must be made on a number of samples or 
pre-production models. The models are then taken 
out to sea for performance tests. Often several modi- 
fications are made in the original design before it is 
acceptable. When a'satisfactory type is finally evolved, 
the manufacturer proceeds with the production of 
the device. Specifications are set up to make the prod- 
uct as much like the samples as possible. To insure 
this, tests must be made on each unit; thus production 
testing involves calibration, albeit in simplified form. 

A production testing program serves two purposes: 
(1) It insures that each product meets certain specifi- 
cations, and (2) it permits quality control, that is, a 
running check on the quality of the manufactured 
products which notes and corrects any deviations 
from the accepted standard. Without regard to the 
detail of the nature of the test or of the device under 
consideration, certain basic requirements must be 
met in order to have a successful production test. The 
procedure must involve a relatively simple technique 
not requiring highly trained personnel. The time re- 
quired for each test must be short. The test should 
not be affected by conditions beyond the operator’s 
control, such as phenomena of noise interference, 
water temperature, etc. 

Production testing of a sonar transducer falls into 
two parts: (1) tests of physical strength, watertight- 
ness, and polarity of electrical elements, and (2) acous- 
tic measurements of directivity, response, and impe- 
dance. This chapter deals only with tests falling in 
the second category. 

In connection with acoustic production tests, lakes 
and rivers are, in general, eliminated from considera- 
tion as testing sites on the following counts: (1) They 
are, as a rule, separated from the factory; (2) the con- 
ditions there may not be sufficiently well controlled 
for routine testing; (3) they may be subject to noise 
from water traffic. An indoor tank in the factory offers 
the best chance of circumventing these difficulties 


while still meeting the requirements of simplicity, 
speed, and sufficient accuracy. It is true that, in a con- 
fined body of water, proximity effects and reflections 
are present and will affect the measurements. On the 
other hand, there are various methods of eliminating 
the effects of these reflections and of correcting for 
proximity effects. (See Chapter 5 for complete discus- 
sion.) These methods will be considered here with 
particular application to the problem of acoustic pro- 
duction testing procedure. 

8 2 PRODUCTION TEST MEASUREMENTS 

The measurement of the acoustic properties of a 
sonar transducer has been discussed in Chapters 4, 5, 
and 6. For a complete description of a sonar trans- 
ducer, it is necessary to know the receiving or trans- 
mitting response as a function of frequency, the im- 
pedance as a function of the frequency, “ and the direc- 
tivity patterns at several frequencies in one or more 
planes, depending on the symmetry of the device. 
Since the test requirements for each of these measure- 
ments are by no means identical, it is necessary to dis- 
cuss the requirements for each measurement sepa- 
rately and to evolve a test procedure which satisfies 
the maximum demand of each test. It should be kept 
in mind, however, that each type of transducer will be 
a special problem and that, in many cases, certain 
measurements may be considerably simplified and 
sometimes even eliminated. 

^ Response Measurements 

For a production test of response, an absolute meas- 
urement is not necessary. The response of the trans- 
ducer can be compared directly with that of a second- 
ary standard, which may be a transducer of the same 
type meeting manufacturing specifications. The cali- 
bration of the secondary standard should be obtained 
from a complete free-field calibration. A relative 

a Impedance measurements have been included in this group, 
since they depend on the acoustic terminating impedance of the 
transducer. 


149 


150 


PRODUCTION TESTING OF SONAR TRANSDUCERS 


measurement of response in a production test per- 
formed in a comparatively small tank may involve a 
smaller testing distance than is required to render 
negligible the various proximity effects. Such a test 
can nevertheless be satisfactory, provided there is a 
definite correlation between comparison measure- 
ments on the standard and on the units under test at 
large and at small distances.^*^ 

Unless proper precautions are taken, response 
measurements made in a tank are not precise because 
of the interference between the direct and reflected 
waves. If the testing distances required because of 
proximity effects are not too large, it may be prac- 
ticable to use a fairly small tank with walls having 
from 15- to 20-db absorption.'^ Such absorption will 
reduce the intensity of the reflected waves sufficiently 
to reduce the error in the axis response to ±1 db 
(under steady-state conditions). If the units to be 
measured are directional, and if directional standards 
are used, the sources of the more bothersome reflec- 
tions may be treated with the recently developed 
“bubble” layer, which provides 5- to 10-db absorption 
and probably renders negligible the overall remain- 
ing reflection interference. (See Chapter 6.) 

In addition to, or in place of, the use of absorb- 
ing walls, electrical methods for eliminating the ef- 
fects of reflections may be employed. These methods 
involve the use of noise or frequency warble, or of 
pulses. The relative advantages of these methods are 
discussed in Chapter 5. Their usefulness depends on 
the nature of the response of the transducer being 
tested, since each method entails a loss of resolving 
power in the curves of continuous- wave response ver- 
sus frequency. For a unit with fairly uniform response 
(Q small), all of these methods are quite satisfactory, 
since little resolving power (RP) is required (RP = Q). 
For a highly resonant transducer, the use of all these 
methods with a tank of given size is limited by re- 
solving power consideration. The resolving power 
will have to exceed Q, and this implies a minimum 
allowable path difference AL between direct and re- 
flected waves, greater than for example, greater 

than 3 meters for / = 25 kc and Q = 50. Regarding 
the relative merits of continuous-wave noise or warble 

b An absorbing tank approximately 3 ft long by li/^ ft wide 
by 2 i/ 2 ft deep has been built by the Bell Telq^hone Labora- 
tories. 35 

c Here c is the velocity of sound, / is the frequency, Q is de- 
fined precisely in Chapters 4 and 5. A derivation and discussion 
of this equation is given in Chapter 5. 


versus pulses, the latter may be considered superior, 
since with their use reflections can be eliminated 
completely from the measurements, while with the 
former the reflections are averaged in and yield a 
time-independent, but not always known, correction. 

Impedance Measurements 

The measured electrical impedance of a transducer 
depends on the terminating acoustic impedance. (See 
Chapters 3, 4, and 5.) This fact deserves consideration 
because the presence of reflected waves incident on 
the face of the transducer constitutes a change in the 
terminating acoustic impedance and so will affect any 
electrical impedance measurement. If the impedance 
measurements are made in a tank with absorbing 
walls, the reflections may be sufficiently small to allow 
the measurements to be made in a conventional way 
with an impedance bridge. 

On the other hand, when the reflections are appre- 
ciable, their effect can be eliminated by pulsing. The 
pulse length is presumably determined by the same 
criterion as in the directivity pattern and response 
measurements (see Section 8.2.3). For pulse measure- 
ments, a wattmeter with a time constant small com- 
pared to the length of the pulse may be used.^ In this 
case the impedance is obtained from voltage, current, 
and power. 

8.2.3 Directivity Pattern Measurement 

The measurement of directivity patterns imposes 
the most severe test requirements. This is chiefly due 
to the fact that measurements of intensity 40 or 50 db 
below the response on the axis must be made in com- 
petition with reflections of the main beam. Even with 
the best of the available absorbing materials, it is im- 
possible to prevent some reflections from interfering 
with the measurement of the directivity pattern of a 
moderately directional transducer. Noise and fre- 
quency warble are not suitable for directivity meas- 
urements because they average in an interfering re- 
flection of unknown magnitude with the direct signal. 
The error introduced thereby may be considerable 
for certain directions, as when a direct signal received 

d The recording wattmeter described in Chapter 6 can be 
used in conjunction with a pulse recording system. Indeed, this 
system has the additional advantage of being able to measure 
impedance at high power levels without unduly heating the 
transducer. 


TANK DESIGN CONSIDERATIONS 


151 


off the axis of maximum response is averaged with a 
reflection incident on the projector axis. 

Pulse measurement, therefore, is the only feasible 
method for obtaining directivity patterns, and even 
this method has its limitations. As noted above in the 
discussion of response, and as described in detail in 
Chapter 5, the pulse method discriminates against re- 
flections by measuring the direct signal before the 
reflection arrives. The time elapsing between the ar- 
rival of the direct and reflected signals is a function of 
the size and shape of the testing tank. The practical 
limits to the maximum allowable size of such a tank 
restrict, in turn, the maximum length of time during 
which the pulse measurement may be made. The 
maximum elapsed time needed for a significant meas- 
urement of the response depends on the transient re- 
sponse of the transducer to a suddenly applied sinu- 
soidal signal, that is, the pulse. For transducers whose 
frequency response characteristic is fairly uniform, 
the response reaches its steady-state value in a short 
time, i.e., a few cycles, and very short pulses and con- 
sequently small testing distances may be used. For 
resonant transducers, the time needed for the response 
to reach its steady-state value is long. In this case, the 
directivity patterns can be measured only by using 
long pulses, and correspondingly long testing dis- 
tances are required. 

The quantitative criterion for the minimum allow- 
able path difference AL between the direct and any 
reflected wave, a quantity which must be just larger 
than the pulse length, has been given above as 
AT > cQlf. A theoretical analysis indicates that a 
much shorter pulse length, and so a much shorter 
minimum path difference, may be used in taking the 
directivity pattern at the resonance frequency of a 
highly resonant transducer. The criterion for AL in 
this case is AL ^ transducer diameter. However, this 
theoretical analysis assumes that the transducer dia- 
phragm moves rigidly, a condition not generally ob- 
tained in practice.® 

The directivity pattern as measured should be a 
close approximation to the directivity pattern that 
would be obtained at large distances. In order to meet 
this requirement, the test distance should be such 
that proximity effects are small, that is, the spherical 
wave correction should be less than a few db. It is 

e See reference 55 for report on investigation of the effect of 
pulse length on pattern for a representative resonant transducer 
(Q ^ 50) operating at the resonance frequency. 


questionable whether patterns taken at a shorter test 
distance can be compared with corresponding pat- 
terns taken on a secondary standard. There is un- 
doubtedly some correlation between the directivity 
patterns of the standard and of the test unit at large 
and at small distances, but the relation is in general 
a very complex one/ 

83 TANK DESIGN CONSIDERATIONS 

Since the pulse method seems best for directivity 
measurements, the specifications for a tank suitable 
for that method will be discussed. Assume that the 
test distance and pulse length are chosen on the basis 
of the foregoing discussion. The absorption of the 
tank walls is of little moment for pulsing. Frequently, 
however, steady-state measurements of response and 
impedance are to be made in the same tank. For these, 
the absorption of the walls should be made as great as 
practicable. Tank walls made of wood or concrete 
offer several db of absorption. If the concrete tank is 
set in the ground, the damp earth provides additional 
acoustic loss on reflection. A steel tank is to be 
avoided, if at all possible, because of the high reflec- 
tion coefficient. A bubble layer may be applied to the 
walls to increase the absorption if it proves necessary. 

The maximum repetition rate of the pulses used 
depends on the reverberation time of the tank. If a 
high repetition rate is desired for ease of measure- 
ment, some wall absorption should be supplied.^ 

While many tank shapes are possible, the simplest 
to build is a rectangular one as deep as it is wide. It 
can be shown that, for a given testing distance and re- 
flection path length, the minimum size is obtained 
when the line joining the projector and hydrophone 
is parallel to the long dimension of the tank. This as- 
sumes that the reflection path lengths from the sides, 
top, and bottom of the tank are equal to those from 
the ends. For this arrangement (Figure 1) the length 
of the tank is given by / = d + AT, where I is the 
length of tank, d is the test distance, and AL is the 
path difference required. 

The width and depth necessary are equal and are 
given by the relation w = \/2dAL H- AL^. 

It may be possible to reduce the width of the tank 
somewhat by the use of completely reflecting baffles 

f For example, in a tank with an average dimension of 15 ft 
and a wall absorption of 3 db per reflection, about 23 milli- 
seconds are needed for the intensity of a reflection to fall 45 db. 


152 


PRODUCTION TESTING OF SONAR TRANSDUCERS 



Figure 1. Optimum test geometry in a simple rectangular 
tank without baffles designed for a given test distance and 
reflection path length. 



OPAQUE 

BAFFLE 


Figure 2. Arrangement of baffles in a rectangular tank 
for the interception of principal reflections. 


placed in such a way as to intercept the reflection 
from the side of the tank. One possible arrangement 
of baffles is shown in Figure 2. This procedure cannot 
be used to eliminate reflections, but it will increase 
the effective path length between them. 

8 4 CHOICE OF SECOND TRANSDUCER 

The choice of the second transducer to be used in 
the acoustic production test measurement depends in 
general on the instrument under test. For receiving 
response measurements the second transducer must 
be a sound source, and vice versa. Its response should 
be fairly uniform over the frequency range being in- 
vestigated, so that it responds rapidly to acoustic and 
electric transients; that is, the time constant of the 
second transducer should be small if it is to be used 
in pulse testing. Also, if its response is uniform, 
slight inaccuracies in frequency will not cause appre- 
ciable errors. The transducer should be fairly stable 
and show only small changes in response with tem- 
perature. 

For continuous-wave noise or warble measure- 
ments, the transducer should be directional. This will 
help to discriminate against reflections coming from 
directions other than that of maximum sensitivity. 
However, since at a given frequency greater directiv- 
ity can be obtained only by increasing the size of the 
transducer, there will be a certain maximum direc- 
tivity beyond which it will be impossible to go. Fur- 
thermore, the proximity effects increase with trans- 
ducer size. 

The NDRC 6B standard projector will be a suit- 
able transducer for most of the cases encountered. 
Any other transducer with similar properties will also 
be suitable. 


8 5 NATURE OF PRODUCTION TEST 

1 he degree of refinement of a production test de- 
pends on the information desired. A calibration test 
system such as that described in Chapter 6 is capable 
of giving a permanent record of any and all charac- 
teristics of a transducer. If the response of a trans- 
ducer at only a few discrete frequencies is needed, the 
test system may be a tank, oscillator, standard trans- 
ducer, and a suitable a-c voltmeter. The response in 
this case is given simply as a meter reading recorded 
by the operator. Likewise, if only a few features of a 
directivity pattern are required, no complicated polar 
pulse recording system is needed. A simple way of 
measuring relative magnitudes of pulses is to use an 
attenuator in the circuit to keep the magnitude of the 
observed pulses constant on a cathode-ray tube screen. 
1 he settings of the attenuator give the relative magni- 
tudes of the pulses. In this way, a directivity pattern 
may be constructed from point by point observations. 
While this procedure is lal^orious, the number of tests 
to be made may not justify a more complicated system. 

d'he pulse method is recommended for determin- 
ing the directivity patterns of large echo-ranging pro- 
jectors. The pulse length used depends on the re- 
sponse characteristic, in the manner described above. 
The tank must be large enough to satisfy proximity 
effect and reflection path length retjuirements. 

For measuring the response of small transducers, 
testing distances may be short, and some sort of tank 
with absorbing walls may be practical. While the 
pulse technicpie is still applicable, continuous-wave 
or noise and warble methods may be simpler and 
yield results within the accuracy desired. 

The absorbent tank described above will also be 
adecjuate for continuous-wave measurements of the 
electric im]>edance of most devices. 


Chapter 9 

ACOUSTIC EQUIPMENT ASSOCIATED WITH 
UNDERWATER SOUND DEVICES: ' 
DOMES AND BAFFLES 

By Henry Primakofj and Joseph B. Keller 


T he Underwater Sound Reference Laboratories 
has calibrated acoustic equipment auxiliary to 
electroacoustic transducers. Among the most impor- 
tant auxiliary equipment tested have been stream- 
lined domes and baffles. 

9.1 DOMES48^2 

In general, a streamlined dome is necessary for an 
echo-ranging or listening device to minimize noise by 
reducing the turbulence and cavitation about its ac- 
tive face arising from its passage through the water. 
The alternative possibility of streamlining the device 
itself has not been widely adopted. 

The dome should be properly streamlined, that is, 
it should be of such a size and shape that turbulence 
and cavitation noises are eliminated, or at least do 
not set in until high speeds of the vessels are reached. 
Further, the dome structure should have sufficient 
mechanical strength to resist the hydrodynamic pres- 
sure and drag forces on it and should be constructed 
from a noncorrosive, sea-resistant material. Finally, 
the dome should be acoustically transparent, causing 
as little disturbance as possible in the magnitude and 
directivity of the response of the enclosed acoustic 
device. 

To be acoustically transparent, the dome must ful- 
fill three requirements. 

First, the dome must introduce only a small trans- 
mission loss. By such a loss is meant the reduction in 
the magnitude of the response of the transducer 
caused by placing it in the dome; usually this is meas- 
ured along the transducer axis. Thus, for an echo- 
ranging projector a 3-db one-way transmission loss 
means a 50 per cent decrease in the pressure ampli- 
tude of any echo received by reflection from a target. 

Second, the dome must introduce no large side 
lobes in the directivity pattern of the enclosed trans- 
ducer. Such lobes may arise as a result of internal 
specular reflections from the dome and cause false 


APPARENT RELATIVE 
BEARING OF TARGET 


/ 

/ 

/ 




/ 

/ 



Figure 1. Specular reflection in domes. 


bearings to be taken in echo ranging or directional 
listening. For example, if a listening or echo-ranging 
device on an antisubmarine vessel is trained in the 
direction of the transducer axis as indicated in Fig- 
ure 1, and an enemy submarine is present in the 
direction of the internal specular reflection, a rela- 
tively strong signal will be received. The vessel may 
then assume that the signal is being received on the 
main lobe, and head into the indicated bearing. Ac- 
tually, in this case, the vessel should head in the 
direction of the specular reflection. 

In addition to introducing one relatively strong 
internal specular reflection, domes also distort the 
transducer directivity patterns by giving rise to vari- 
ous second order effects such as multiple reflections. 
Multiple reflections introduce additional side lobes 


153 


154 


ASSOCIATED EQUIPMENT: DOMES AND BAFFLES 


in the pattern, increasing such effects as the rear re- 
sponse. However, if the domes are well designed 
acoustically, these additional side lobes are usually 
over 20 db down with respect to the main lobe. For 
this reason it is generally desirable to distribute the 
sonic energy contained in a single internal specular 
reflection among many directions unless the specu- 
larly reflected beam can be intercepted and further 
subdivided or absorbed.-^^’''*^ Furthermore, the enclo- 
sure of a transducer in a dome should not appreciably 
alter the width of the main lobe or increase the mag- 
nitude of the side lobes already present in the trans- 
ducer patterns. (These two effects are quite small in 
well-designed domes.) 

Third, the enclosure of a transducer in a dome 
should not appreciably alter the radiation impedance 
of the transducer. (Impedance change is usually very 
small in well-designed domes.) 

Acoustical disturbances introduced by domes, such 
as specular reflections and transmission losses, are in- 
terrelated.Jn general, a dome which introduces small 
transmission losses also causes small specular reflec- 
tions. (A quantitative relation between the two is 
given later.) Moreover, because the change in the 
radiation impedance of the transducer is small, its 
total power output is unaffected by enclosing it with- 
in a dome; also, true absorption of sound within the 
dome wall is negligible for metal domes. As a result, 
the energy which is removed by the dome wall from 
the impinging transducer beam and which consti- 
tutes the transmission loss is redistributed in direc- 
tions other than the original direction of incidence; 
in particular, the major portion of this energy is con- 
centrated into the direction of specular reflection." 
This redistribution has the effect of increasing the 
value of the directivity factor 8. (The directivity fac- 
tor is related to the directivity index by the expression 
A = 10 log 8. See Chapters 3 and 4.) Thus, for an 
echo-ranging projector:^’^ 


pi = axis pressure output of dome-enclosed projec- 
tor at 1 meter, 

8 = directivity factor of bare projector, 

8' = directivity factor of dome-enclosed projector, 
Po = density of water, and 
Co = velocity of sound in water. 

Therefore, the one-way transmission loss TL intro- 
duced by the dome, defined by 

TL = 20Iog^, (2) 

pi 

is, from equations (1) and (2) 

rL = 201og|'. (3) 

Thus, the expression TL is a measure of the change 
in the transducer directivity index introduced by the 
dome. 

Expressions may now be obtained theoretically for 
the magnitudes of both the transmission loss and the 
specular reflection induced by a dome of given mate- 
rial, wall thickness, and dimensions, on an enclosed 
transducer of given frequency, directivity, and posi- 
tion within the dome.^® These expressions are found 
by determining the effect of dome enclosure on the 
sound field of a transducer; they agree generally with 
experimental tests on domes performed by USRL.*^ 
The expression for the transmission loss (and so by 
equation (3) for the dome-induced change in direc- 
tivity index) is: 

Tr^I01og[l + (^y]. (4) 

where 

pi = density of dome wall material. 


( 1 ) 

Po^o Po^o 

where 

P = acoustic power output of echo-ranging pro- 
jector, 

pi = axis pressure output of bare projector at 1 
meter. 


a Thus, the magnitude of the additional side lobes introduced 
by the dome into the directivity pattern, for example, the addi- 
tional rear response, increases as the transmission loss increases. 


Po = density of water, 
d = thickness of dome wall. 



Aq, Co = wave length and sound velocity in water, 
and 

/ = frequency, 
b See STR Division 6, Volume 11. 


DOMES 


155 


It is important to note that the transmission loss is 
determined solely by the thickness and density of the 
dome wall and by the frequency of the transducer and 
is independent of the transducer directivity, of its 
position within the dome, and of the dimensions of 
the latter.*^ Thus in particular, with d, pj, and / fixed, 
the dome may be made of any size and shape, for ex- 
ample, elongated for streamlining purposes, without 
the transmission loss being affected. Regarding nu- 
merical values, equation (4) predicts, for example, 
that a 50-mil steel dome at a frequency of 25 kc will 
have a transmission loss = 1 db.^®-^- 

It will be recalled that the transmission loss is a 
measure of the total amount of energy removed by 
the dome from the main beam and diverted into 
other directions. The fraction of this energy reap- 
pearing in the direction of specular reflection de- 
pends on the specular reflection coefficient R. This 
coefficient gives the ratio of the dome-enclosed trans- 
ducer response in the direction of specular reflection 
from the dome surface to that in the direction of the 
transducer axis. R depends on the transducer fre- 
quency and on the thickness and density of the dome 
wall. But R is also determined by the directivity of 
the transducer, by its position in the dome, and by the 
dome dimensions. Thus, 

when /fort2/4 < A 


^ 20 log 


2po 


cos y 


(5a) 


when k^a-jA > A 


7 ? = 20 




(5b) 


where 


•Vii = 


/^o^/-secy D \27rn^ 
2Rn 


LM KD ’ 


//fo«“COS-y /27rrt2 

^ \ — ^R ~ '\1 \ D 


r(x) ^ 1 

.V .Y 


nr- 

e dv 


domes, 

= 0 for straight-sided domes; 

^ 1 for Y < < 1 , 

= —^ — for X >> 1, 

V2 X 

< 1 for all X. 


Here a is the acoustic radius of the transducer;^^ R , | 
and R^ are the principal radii of curvature in the 
horizontal and vertical planes of the dome wall at the 
point of its intersection with the transducer axis; L 
and D are the maximum linear and transverse dimen- 
sions of an approximately ellipsoidal, torpedo-shaped 
dome or of a straight-sided dome with approximately 
elliptical cross section; A is the distance from the 
transducer diaphragm to the dome wall for the train- 
ing position under consideration; and y is the angle 
of incidence of the enclosed transducer’s beam on the 
dome wall.'*^-*'*- 

It is now seen from an examination of equation (4) 
that the transmission loss of the dome is minimized 
when the thickness of the dome wall, the density of 
the dome material, and the frequency of the enclosed 
projector are as small as possible. These quantities 
should therefore be chosen accordingly, consistent 
with the mechanical strength of the dome wall, the 
seaworthiness of the wall material, and the directiv- 
ity of the projector. In particular, the dome designer 
should use light metals such as aluminum and pos- 
sibly organic materials like rubber;® shapes of maxi- 


c The transmission loss is also roughly iiulependent of llie 
angle of incidence of the enclosed projector beam on the dome 
wall for angles of incidence less than 50°; for greater angles of 
incidence the transmission loss increases rapidly, because of pro- 
pagation with appreciable amplitude of transverse elastic waves 
in the dome wall, 

<1 The acoustic radius a is the radius of the rigid circular 
piston moving an infinite baffle having a beam width 20 and 
a directivity index A ecjual to that of the actual projector 
[1.42 sin 0 = 0.61c//fl, A s 20 log ic/2,irj(i), see Chapter 4]. 


e Thus aluminum, various plastics, and stiff rubber strength- 
ened mechanically by an expanded metal structure have all 
been used because of their relatively small density to minimize 
dome transmission losses and specular reflections. The seaworth- 
iness of these materials, especially the first two, is open to ques- 
tion. It is claimed that aluminum easily corrodes in sea water; 
proper treatment of the metal may, however, render it salt 
water resistant.40 Also, plastics are subject to aging and tem- 
perature effects. A stiff rubber structure may, however, turn out 
to be cjuite satisfactory 


156 


ASSOCIATED EQUIPMENT: DOMES AND BAFFLES 


mum strength for given thickness should also be cho- 
sen, that is, the dome should be torpedo-shaped rather 
than straight-sided. Added strength can be obtained, 
if necessary, by expanded metal or possibly by corru- 
gated sheet construction in preference to increased 
wall thickness. 

It is seen from equations (5) that the specular re- 
flection is not only minimized by minimizing the 
transmission loss but also is least for a given transmis- 
sion loss when the projector is as directive as possible, 
the transverse dimensions of the dome are no larger 
than required to accommodate the projector, and the 
dome surface is as highly curved as possible, particu- 
larly in the vertical cross section. 

In this last respect the doubly curved torpedo- 
shaped domes again have a decided advantage over 
the singly curved straight-sided domes. For example, 
at 25 kc, for a 40-mil thick torpedo-shaped dome of 
intermediate dimensions, the specular reflection is 
about 20 db below the main beam; for a straight-sided 
dome of the same sound window thickness and the 
same dimensions the specular reflection is only 10 db 
below the main beam.^® In addition, the torpedo 
shapes have a considerable advantage from the stand- 
point of mechanical strength and streamlining.^^ Ex- 
cessive noises due to eddying turbulence and to cavi- 
tation set in at rnuch higher speeds for doubly curved 
torpedo-shaped domes than for singly curved straight- 
sided domes. Also, much of the self or ship's noise 
within a dome at supersonic frequencies, for a de- 
stroyer moving at relatively high speeds, arises from 
streaming turbulent water, particularly in instances 
where bubbles detached from the prow strike the 
dome and set the dome shell into vibration. This 
bubble noise is probably smaller for torpedo-shaped 
than for straight-sided domes because of the small 
front area and greater streamlining of the latter. 

In regard to the effect of modern thin-wall (20 to 
40 mil) stainless steel domes on echo ranging and lis- 
tening, it may be shown that the decrease in the 
signal and the increase in the noise due to the dome 
transmission loss and the associated change in the 
directivity index affect detection ranges for various 
types of targets by relatively small amounts. A side 
lobe introduced by specular reflection in the directiv- 
ity pattern may not be troublesome at first contact 
(except for a 60-mil, straight-sided dome), but it may 
give false bearings and thus make it difficult to regain 
contact at short range. 1 he use of 20- to 40-mil stain- 
less steel torpedo-shaped domes, with specular reflec- 


tions 25-20 db below the main beam, overcomes this 
difficulty and in addition minimizes any mutual in- 
terference between beams from different craft. 

92 BAFFLES68 

The self noise picked up by a transducer on a mov- 
ing vessel is due in part to turbulence and cavitation 
at the propeller screws; therefore, it might be ex- 
pected that this noise is directional and has a maxi- 
mum in the direction of the screws. For small vessels 
this is the case, necessitating additional acoustic 
shielding of the screws; on the other hand, on larger 
vessels the hull partially shields the transducer from 
the screws. The available information on the extent 
of acoustic shielding by the hull of various types of 
craft is meager; from the evidence at hand it seems de- 
sirable to provide additional shielding both on small 
and large vessels. On vessels where the self noise has a 
maximum in the direction of the screws, arising pre- 
dominantly from propeller motion, such shielding 
will be advantageous. Even where noise is apparently 
nondirectional, a shield may lower the average noise 
level. f 

A common method of acoustic shielding is to place 
a baffle inside the dome between the transducer and 
screws. There are two possible disadvantages in this 
method: First, sound incident on the transducer side 
of the baffle may be reflected to the transducer, in- 
creasing the noise level or giving spurious indications. 
This difficulty is eliminated by covering the front of 
the baffle with an absorbing material. The second dis- 
advantage is the prevention of echo ranging or listen- 
ing in the sector subtended by the baffle. Usually this 
is not a serious objection, since the baffle is always to 
the rear of the transducer and screw noise plus wake 
make it useless, in general, to listen or range toward 
the rear. In some cases the noise or echo of the sub- 
marine may be intense enough to be detectable in the 
rear sector; in this case the baffle actually prevents 
detection. This disadvantage may be great enough to 
offset any of the possible advantages of a baffle; the 
difficulty can be overcome by the use of an additional 
transducer without a baffle to sweep the rear sector. 


f Nondirectional or isotropic self noise has been observed on 
large antisnbmarine craft with dome-enclosed transducers 
shielded by baffles from the screws. Whether the noise still ap- 
pears isotropic in the absence of the baffle is not known; if it 
does, the supposition that the noise arises largely from bubbles 
striking the dome obtains support.^Oa 


BAFFLES 


157 


The effectiveness of a baffle depends largely upon 
its material, its size and thickness relative to the sound 
wave length, and its location with respect to the trans- 
ducer. When the problem of sound propagation past 
a baffle is considered, it is found that, for a plane wave 
incident on one side of the baffle, the pressure on the 
other side is due to both a transmitted wave and a dif- 
fracted wave. A sound shadow is formed behind the 
baffle only if the transmitted wave is small. The deci- 
bel ratio of the incident intensity to the transmitted 
intensity (through an infinite plane baffle) is called 
the transmission loss TL^ and is given by Rayleigh’s 
formula."^ 


TL 




piCi Y . ^ 

) sin2 

Po^o/ 


27rd l 

Ai J 


(<'•) 



Figure 2. Transmission loss of baffles. 


where 

TL = transmission loss, 

Pq = density of water, 

Co = sound velocity in water. 
Pi = baffle density. 


sound intensity is zero is never formed behind the 
baffle. For incident plane waves of wave length A in 
water, diffraction around the baffle edge limits the 
shadow to a length of approximately ^^/(ttA), where 
A is the area of the baffle projected on the wave front 
and ^ is a numerical factor ^ 1. (For a circular baffle 


Cl = sound velocity in baffle material, 
d = baffle thickness (or average thickness), and 
Ai = sound wave length in baffle material. 

This expression for TL versus d/Ai is plotted in fig- 
ure 2 for a plane parallel baffle of air (scale 1) and 
steel (scale 2) in water. It can be seen that a plate with 
a large loss at one frequency will also have a large loss 
over an extensive range of frequencies. The baffle 
thickness is not critical as long as it is not equal to a 
value appropriate to resonance transmission, that is, 
when 2d/Ai is not equal to an integer. It is, therefore, 
not difficult to obtain a baffle with a large transmis- 
sion loss; for example, a steel plate about 1 inch thick 
has a transmission loss of roughly 20 db at 24 kc. 
Thus, as far as transmission loss is concerned, such 
steel baffles are adequate; baffles containing air pock- 
ets are generally even more effective, the loss of a 
0.2-inch thick air baffle at 24 kc being approximately 
60 db. 

Even when transmission loss is large, the infinitely 
long shadow of geometrical acoustics in which the 

g Equation (6) reduces to equation (4) if 27rd/\j « 1 and 
PiT/Po^o ^ conditions always satisfied by the thickness and 
material of dome walls in current use. 


PLANE WAVE 
INCIDENT SOUND 



AIR BAFFLE 


PLANE WAVE 
INCIDENT SOUND 


BAFFLE 



INTENSITY OF SHADING REPRESENTS SOUND SHADOW, 

(le. heavy SHADING CORRESPONDS TO LOW SOUND PRESSURE ) 




iiRK .4. Soiinrl nressiire distrilnition 




158 


ASSOCIATED EQUIPMENT: DOMES AND BAFFLES 






PERPENDICULAR DISTANCE FROM THE BAFFLE AXIS IN INCHES 


VARIATION OF SOUND PRESSURE BEHIND 18” CIRCULAR BAFFLE 
WITH PERPENDICULAR DISTANCE OFF THE BAFFLE AXIS 


THE SMOOTH LINE ABOVE EACH PATTERN GIVES THE SOUND PRESSURE WITHOUT THE BAFFLE. 

THE VERTICAL DOTTED LINES GIVE THE THEORETICAL POSITION OF THE "EDGES OF THE BRIGHT 
SPOT"i.e.,OF THE PRESSURE MINIMA ON EITHER SIDE OF THE CENTRAL PRESSURE MAXIMUM. 

THE CROSS GIVES THE THEORETICAL VALUE p 

^AXIS’ 

THE DISTANCE BEHIND THE BAFFLE AT WHICH THE PRESSURE IS MEASURED IS X; f IS THE 
FREQUENCY IN KC. 


Figure 4A. Experimental variation of pressure behind baffles. 



BAFFLES 


159 



>- 

(T 

< 

a: 

K 

m 

tr 

< 

(/) 

> 

m 

o 

z 

LJ 


< 

CD 


X 

liJ 

CD 

liJ 

a: 

3 

(D 

(/) 

liJ 

q: 

CL 











1 

Lad. 









1 



1 

1 

1 




H 


X*96'inche; 
f*IO KC 






/ 

EDGE OF 

1 

GEQMETRICA 

L 


L_ 

1 

_1L 

1 



.J 


SHACX)W 

_LJ L 



20 15 ioj 5 0 5 JlO 15 20 





PERPENDICULAR DISTANCE FROM THE BAFFLE AXIS IN INCHES 


VARIATION OF SOUND PRESSURE BEHIND 14" a 18" CIRCULAR AND RECTANGULAR 
BAFFLES WITH PERPENDICULAR DISTANCE OFF THE BAFFLE AXIS 


THE SMOOTH LINE ABOVE EACH PATTERN GIVES THE SOUND PRESSURE WITHOUT THE BAFFLE. 

THE VERTICAL DOTTED LINES GIVE THE THEORETICAL POSITION OF THE "EDGES OF THE BRIGHT 
SPOT";.e.. OF THE PRESSURE MINIMA ON EITHER SIDE OF THE CENTRAL PRESSURE MAXIMUM. 

THE CROSS GIVES THE THEORETICAL VALUE n 

^^AXIS 

THE DISTANCE BEHIND THE BAFFLE AT WHICH THE PRESSURE IS MEASURED IS X; f IS THE 
FREQUENCY IN KC. 


Figurk 4Ii. Experimental variation of pressure behind baffles. 



160 


ASSOCIATED EQUIPMENT: DOMES AND BAFFLES 


calculations indicate that p = 1.5.)^® The cross sec- 
tional area of the shadow is roughly A immediately 
behind the baffle and decreases to zero at a distance 
approximately (SAKttX) from the baffle. But even in 
the shadow region the sound pressure is not zero, the 
exact pressure distribution in the shadow depending 
upon the size, shape, and material of the baffle. Since 
the shadow of a circular disk baffle (air or steel) has 
been studied both theoretically and experimentally, 
it will be used as an example. Because of the sym- 
metry of the circular baffle, the shadow is symmetrical 
about the axis. Thus, Figure 3, which shows the 
shadow region in a plane containing the axis, can be 
revolved about the axis to give a three-dimensional 
picture of the shadow region. The interference maxi- 
mum on the axis— the “bright” spot— is due to the spe- 
cial symmetry of the circular baffle and would not be 
present if the baffle were another shape, such as rec- 
tangular.^^ On the other hand, the converging of the 
outer surface of the shadow due to the increase in size 
of an illuminated annular ring always occurs. 

In addition to the bright spot, there are alternate 
bright and dark rings around the axis in any plane 
parallel to the baffle. Figure 4 shows the variation of 
pressure obtained experimental ly^ in the shadow of 
18-inch and 14-inch diameter circular and 4x8 foot 
rectangular air-filled baffles along a line through the 
axis and parallel to the baffle diameter, at various 
distances from the baffle with 10-, 20-, and 40-kc 
sound. These patterns are in general agreement with 
theoretical expectations.^ 

Thus, for a baffle to be at all effective in shielding 
a transducer with a fairly large active face, that is, an 
echo-ranging projector, the baffle must have a large 
transmission loss (for example, ^ 25 db), must be 
considerably greater in area than the transducer, and 
must be placed appreciably nearer to the latter than 
A /(ttA). To make this statement more precise, the ef- 
fect of interposing a baffle between a transducer and 
a source is considered for two cases: 


In Uie rectangular case, the energy previously concentrated 
by the circular baffle into the axis bright spot would he more or 
less uniformly distributed among several interference maxima 
within the shadow zone. Because of the “energy redistribution” 
the shielding effects of circular and rectangular baffles of com- 
parable size on actual transducers (for example, the rather large 
echo-ranging projectors) are, at the distances used in practice, 
roughly equivalent. 

i These patterns were obtained at the Orlando test station of 
USRL. The sound field was measured with a small size hydro- 
phone so that the pressure at individual jjoiuts iu space was 
obtained. 



Figure 5. Decrease in front transducer response due to 
baffle. 


1. The transducer faces the source: front trans- 
ducer response. 

2. The transducer faces away from the source: rear 
transducer response. 

Case 2 is of practical interest, corresponding to the de- 
crease in the rear response due to the baffle. Quan- 
tities applicable to cases 1 and 2 respectively are de- 
fined as follows: 

SR = change in front transducer response due to 
baffle interposition, 

8R' = change in rear transducer response due to 
baffle interposition. 


j Thus, the variation of pressure p (within the shadow zone) 
with perpendicular distance y from the baffle axis, and at a dis- 
tance X behind a circular baffle, is 


P = /'axlslo [ ^ 


A 


\ "V 7rX^ + 7ry^ + T 


“Pinef" s'eel baffle 


[ /Vx^-f rt^ + y^-xM 


= p 


inc 


. for air baffle 


+ i 


for y not too close to the shadow boundary = zero order Bes- 
sel function). See reference 68, equations (23) and (24). The theo- 
retically expected values of and of the positions giving the 
edges of the bright spot are indicated on the diagram. 


BAFFLES 


161 



02468 10 246 8 10 

TRANSDUCER BAFFLE SEPARATION IN WAVE LENGTHS 


Figure 6A. Decrease in rear transducer response due to baffle. 


Consider first SR^ Figure 5 gives an experimentally 
obtained plot of SR versus separation between baffle 
and transducer for jilane waves of various frequencies 
incident upon the former.*^ The baffle was air-filled, 
circular in shape, and had an 18-inch diameter; the 
transducer was also circular and had a 14-inch geo- 
metrical and a 10-inch acoustic diameter. It is seen 
that, in general, SR decreases with increasing separa- 
tion; the values of SR, particularly those at the lower 
frecjuencies, are in rough agreement with theory.^ 
Consider now the effect of interposing a baffle on 
the rear response of the transducer. Figure 6 (A, B, C) 
gives theoretically obtained plots of 8R' versus sepa- 
ration between baffle and transducer. The theory and 
the plots here reproduced indicate that an air-filled 


baffle with a transmission loss ^ 25 db decreases the 
transducer rear response by 15-25 db provided that 
the transducer-baffle separation x is considerably less 
than the smaller of the two critical lengths, 7rrt5/ 1.2A 
and 8[(rt — b)-/X + (« — 5) + (A/4)], where a is the 
radius of the baffle and b the acoustic radius of the 
transducer.*" The first critical distance corresponds 
to the bright spot covering the rear of the projector, 
the second, to the annular ring on the rear making an 
important contribution. 

It is seen from the above relations that for a = b, 
that is, for a transducer of the same size as the baffle, 
the baffle-transducer separation x must be less than 
2 A to have any shielding at all. On the other hand, 
for a = b, and x as small as A, the relations given are 


k The data were obtained at the Orlando station of USRL. 
1 See reference 68, equation (44c): 


dR 


0.06 X 


+ 



b — a — 





where X is the wave length, a the radius of the baffle, b the acous- 
tic radius of the transducer, and x the transducer-baffle separa- 
tion (air-filled baffle). Equation (44c) does not include the effect 
of the transmitted wave. 

The plots in Figure 6 are based on equation (47) of reference 
68: SR'= [x/(ab/\) 7i (2.4Va- x^/rt) ] for an air-filled baffle 
(/^ = first order Bessel function; x « Trab/\.2 \ and 8 [(« — by/X 
+ (a — b) + X/4]. Equation (47) and the plots of Figure 6 do 
not include the effect of the transmitted wave. 



162 


ASSOCIATED EQUIPMENT: DOMES AND BAFFLES 



02 468 10 24 68 10 

TRANSDUCER BAFFLE SEPARATION IN WAVE LENGTHS 


Figure 6B. Decrease in rear transducer response due to baffle. 


no longer strictly applicable, and general considera- 
tions indicate that no shielding exists even for x ^ X. 
On the other hand, Figure 6 indicates that for a-b » X 
the shielding is more or less independent of the exact 


value of a-b and not too sensitively dependent on x 
as long as the latter is less than the smaller of the two 
critical lengths given above. However, with air-filled 
baffles the shielding is greatest for small separations. 



FifiURE 6C. Decrease in rear transducer response due to baffle. 


BAFFLES 


163 



180* 



Figure 7. Change in response of dome-enclosed trans- Figure 8. Attenuation versus frequency for different sized 

ducer due to interposition of baffle. bubbles. (R = bubble radius.) 


F'ew measurements have been made on the effect of 
a baffle on the rear response of a transducer, particu- 
larly for transducers enclosed in domes. Figure 7 
shows the results of one such measurement. Referring 
to this figure it is seen that for angles between the 
transducer and dome axes varying from 90-270 de- 
grees (180 degrees corresponds to the transducer fac- 
ing away from the source) a 34x24-inch rectangular 
and dome-enclosed baffle gives an average decrease of 
only a few decibels in the response of a dome-enclosed 
transducer 20 inches away compared to the response 
of a bare projector at the same distance with no baffle 
interposed. These measurements, however, also in- 
clude the detrimental effect of reflections from the 
dome wall which usually increase the response in the 
90 to 270-degree sector by 5-15 db. Thus, in this case, 
the effect of the baffle more or less cancels that of the 
dome. Further experiments are particularly desired 
which will compare the rear response (angles of 90- 
270 degrees) of dome-enclosed and of bare transducers 
with and without a baffle. 

Summing up, it has been shown that the diffraction 
of sound around, rather than its transmission through. 


the baffle limits the latter’s effectiveness. Both with 
regard to transmission loss and to diffraction, air- 
filled baffles are superior to steel. To minimize the dif- 
fraction effect, the baffle should be appreciably larger 
than the transducer and should be placed as close as 
possible to it. The baffle-transducer distance should 
always be considerably less than the smaller of the 
two critical lengths given above. 

Bubble Screens 

ft has been found that air bubbles resonate in water 
at frequencies depending on their size and that at and 
near that frequency they are very effective scatterers 
and absorbers of sound. When, then, a layer of such 
bubbles is inserted in the sound field, it offers very 
high attenuation. Such a layer may be used as a baffle 
and is called a bubble screen. 

The propagation of sound through water contain- 
ing bubbles has been studied by USRL both theoreti- 
cally and experimentally."^4,.-,6 xhe attenuation per 
centimeter thickness of a bubble screen varies directly 
as the number of bubbles per cubic centimeter. Fig- 


164 


ASSOCIATED EQUIPMENT: DOMES AND BAFFLES 


lire 8 shows the attenuation versus frequency charac- 
teristics for different sizes of bubbles. 

The use of bubble screens offers interesting possi- 
bilities, especially since, at least theoretically, it is 
possible by this means to intercept sound waves of 
certain frequencies while others are transmitted 
freely. The design and control of all these factors has 
not been fully worked out, but air bubbles fixed in 
space by means of an enclosing material are being 
used generally in sonar work. For instance, the effec- 


tiveness of air cell rubber as an acoustic shield de- 
pends on this principle. 

Bubbles also have absorptive properties which are 
used in a bubble layer recently developed by the Mas- 
sachusetts Institute of Technology. This material 
has been applied by USRL as a lining for the high- 
pressure tank in order to obtain sound absorption at 
the walls. (See Chapter 6.) It is applicable for testing 
tanks generally and should find extensive use in pro- 
duction testing. (See Chapter 8.) 


GLOSSARY 


ACOl'STIC AXIS. Reference line adopted in transducer cali- 
bration, usually the direction of niaximiun response. 

ADP. Ammonium dihydrogen phosphate crystal having marked 
piezoelectric properties. 

.\/S. .\ntisid)marine. 

B. AFFLE. A shield used to modify an acoustic path. 

BATHYTHERMOGRAPH. .\n instrument which records the 
temperature of sea water as a function of depth. 

BDI. Bearing deviation indicator. 

BTL. Bell Telephone Lahoratories. 

C. WIT.ATIOX. The formation of vapor or gas cavities in water, 
caused hy sharp reductions in local pressure. 

CREST F.ACTOR. In this volume, \/2 times the ratio of peak- 
to-rms pressure of an acoustic wave. 

CRYSTAL TRANSDUCER. A transducer which utilizes piezo- 
electric crystals, usually Rochelle salt, ADP, tpiartz, or tour- 
maline. 

DDL Depth deviation indicator. 

DIRECTI\’ITY INDEX. A measure of the directional prop- 
erties of a transducer. It is the ratio, in dh, of the average 
intensity, or response, over the whole sphere surrounding the 
projector, or hydrophone, to the intensity, or response, on 
the acoustic axis. 

DOME. A transducer enclosure, usually streamlined, used with 
echo-ranging or listening devices to minimize turhulence and 
cavitation noises arising from the passage of the transducer 
through the water. 

ECHO REPEATER. Artificial target, used in sonar calibration 
and training, which returns a synthetic echo hy receiving, 
amplifying, and retransmitting an incident ping. 

ERSB. Expendable radio sono buoy. 

HUSL. Harvard Underwater Sound Laboratory. 

HYDROPHONE. An underwater microphone. 

HYDROPHONE, VELOCITY TYPE. A pressure-gradient hy- 
drophone. 

MAGNETOSTRICTION EEEECT. Phenomenon exhibited hy 
certain metals, particularly nickel and its alloys, which change 
in length when magnetized, or, (V^illari effect) when mag- 
netized and then mechanically distorted, undergo a corre- 
sponding change in magnetization. 

MIT-USL. The Massachusetts Institute of Technology Under- 
water Sound Laboratory. 

NDRC. National Defense Research Committee. 


OSRD. Office of Scientific Research and Development. 

PIEZOELEC ERIC EEEECT. Phenomenon, exhibited hy cer- 
tain crystals, in which mechanical compression produces a 
potential difference Between opposite crystal faces, or, an 
applied electric field produces corresponding changes in di- 
mensions. 

PING. .Acoustic pulse signal projected hy echo-ranging trans- 
ducer. 

PPL Plan position indicator. 

PRESSURE-GRADIENT TRANSDUCER. Transducer, such 
as a moving-rihhon hydrophone, in which the moving ele- 
ment responds to pressure difference rather than to pressure. 

PROJECTOR. An underwater acoustic transmitter. 

R.ADIO SONO BUOY. .A buoy listening device that contains a 
hydrophone for receiving target signals and a radio trans- 
mitter for relaying the signals to patrolling air or surface 
craft. 

R.ANGE R.ATE. Rate of change of range between own ship 
and target. 

RE.AR RESPONSE. The maximum pressure within ±60 de- 
grees from the rear of the transducer in dh relative to the 
pressure on the acoustic axis. 

ROCHELLE SALT. Potassium sodium tartrate (KNaC^H^O^. • 
4H.,0) piezoelectric crystal used in sonar transducers. 

SC.ANNING SON.AR. Echo-ranging system in which the ping 
is transmitted simidtaneously throughout the entire angle to 
he searched, and a rapidly rotating narrow beam scans for the 
returning echoes. 

SEARCHLIGHT-TYPE SONAR. Echo-ranging system in 
which the same narrow beam pattern is used for transmission 
and reception. 

SONAR. Generic term applied to methods or apparatus that 
use sound for N.Avigation and Ranging. 

SPTU. Split projector test unit. 

1 R.ANSDUCER. Any device for converting energy from one 
form to another (electrical, mechanical, or acoustical). In 
sonar, usually combines the functions of a hydrophone and a 
projector. 

USRL. Underwater Sound Reference Lahoratories. 

X-CUT. A cut in which the electrode faces of a ])iezoelectric 
crystal are perpendicidar to an X or electrical axis. 

Y-CUT. A cut in which the electrode faces of a piezoelectric 
crystal are perpendicular to a Y or mechanical axis. 


165 



BIBLIOGRAPHY* 


Numbers such as l)iv. 6 553.1-Ml indicate that the document listed has been microfilmed and that its title appears in the 
microfilm index printed in a separate volume. For access to the index volume and to the microfilm, consult the Army and 
Navy agency on the reverse of the half-title page. 


1. The 5 -A Impedance Bridge, Ludwig E. Herhorn, Bell Sys- 
tem Practices, Section E 10-517, B EL, May 1941. 

Div. 6-553-Ml 

2. Hydrophonic Studies, Proposed Calibration of Electro- 

Acoustic Transducers for Hydrophonic Systems, Eginhard 
Dietze, NDRC C4-sr00-004, Report 2420, B EL, Aug. 23, 
1941. Div. 6-552-Ml 

3. Preliminary Hydrophone Calibrations, Frank F. Romanow, 
Eginhard Dietze, Report 2420, BTL, Nov. 1, 1941. 

Div. 6-554-Ml 

4. Preliminary Calibration of Hydrophones, Eginhard Dietze, 

Report 2420, BTL, Nov. 8, 1941. Div. 6-554-M2 

5. Preliminary Calibration of Condenser Type Hydrophone 

NOCT Xo. 2, Frank H. Graham, Report 2420, BTL, Nov. 
21,1941. Div. 6-554-M3 

6. Hydrophonic Work on Case 23211, William H. Martin, 

BTL, Dec. 4, 1941. Div. 6-554- M4 

7. Preliminary Calibration of Hydrophones, Erhard Hart- 
mann, Report 2420, BTL, Dec. 20, 1941. Div. 6-554-M5 

8. QC Projector Water-Xoise Measurements on the USS Rath- 

burne, Erederick A. Everest, David J. Evans, IICDWR, Feb. 
24, 1942. Div. 6-555-M2 

9. A Primary Standard Pressure Gradient Hydrophone, OSRD 
456, NDRC C4-sr212-058, BTL, Mar. 2, 1942. 

Div. 6-553.1 -Ml 

10. Calibration of Hydrophones and Projectors, Thomas H. Os- 
good, CLDWR-NLL, Mar. 6, 1942. Div. 6-554-M9 

11. Analysis of Calibration Data of Magnetostriction Echo- 
Ranging Equipment, Walter D. Goodale, Jr., Eginhard 
Dietze, Report 2420, BTL, Mar. 18, 1942. Div. 6-554-MlO 

12. Analysis of Measurements on Crystal (JK) Echo-Ranging 

Projector, Walter D. Goodale, Jr., Eginhard Dietze, Report 
2420, BTL, Mar. 27, 1942. Div. 6-554.1-Ml 

13. Calibration of Electro-Acoustic Transducers for Hydro- 

phonic Systems, Eginhard Dietze, NDRC C4-sr212-101 , Re- 
port 2420, BTL, Apr. 17, 1942. Div. 6-554-Mll 

14. Directivity Patterns of Sound Sources, Walter O. Pennell, 

Malcolm H. Hehh, and others, OSRD 706, NDRC C4-sr287- 
089, HUSL, Apr. 29, 1942. Div. 6-551 -M2 

15. Test on Model of Working Standard Projectors for 25-100 

KC Erequency Range, Prank H. Graham, Eginhard Dietze, 
USRL, May 25, 1942. Div. 6-553.2-M2 


* BTL Bell Telephone Laboratories, Inc. 

CUDVV'R-NLL Columbia University Division of War Research at 
the U. S. Navy Underwater Sound Laboratory. 

CUDWR-SSG Columbia University Special Studies Group. 
HUSL Harvard Underwater Sound Laboratory. 


16. A Subaqueous Projector for Hydrophone Calibrations in 
the Audible Erequency Range, Reginald L. Jones, OSRD 
705, NDRC C4-sr212-103, BTL, June 1, 1942. 

Div. 6-553.2-M3 

17. Properties of Acoustic Screens for Underwater Applications, 

Leslie L. Poldy, OSRD 745, NDRC C4-sr20-146, USRL, July 
20, 1942. Div. 6-552-M2 

18. Reverberation in Echo Ranging, Part I, General Principles, 
4Villiam V. Houston, Thomas H. Osgood, OSRD 807, 
NDRC C4-sr20-149, CUDWR-SSG, July 28, 1942. 

Div. 6-520-Ml 

19. Absolute Efficiency of Projectors and Hydrophones, Egin- 

hard Dietze, OSRD 774, NDRC C4-sr20-150, USRL, Aug. 
3,1942. Div. 6-551-M3 

20. The Absolute Efficiency of a Device Used as a Projector and 

as a Hydrophone, Eginhard Dietze, OSRD 811, NDRC 
C4-sr20-197, USRL, Aug. 18, 1942. Div. 6-551 -M4 

21. Preliminary Measurements on the Acoustic Properties of 

Disturbed Uater, Eginhard Dietze, NDRC C4-sr20-205, 
USRL, Sept. 7, 1942. Div. 6-540.3-Ml 

22. Hydrophone Calibrating Equipment. Specification, Roland 
C. Quest, Report G12/4121, CUDWR-NLL, Sept. 15, 1942. 

Div. 6-553-M3 

23. The Calibration of Toroidal Hydrophones in a Canvas 
Tank, Edward Cerjuoy, NDRC C4-sr20-344, Report 
D22.2/3935, CUDWR-NLL, Sept. 23, 1942. 

Div. 6-554.2-M9 

24. A Standard Crystal Hydrophone, OSRD 955, NDRC C4- 

sr212-507, BTL, Oct. 1, 1942. Div. 6-553. 1-M2 

25. Calculated and Observed Speeds of Cavitation about Two- 

and Three-Dimensional Bodies in Hhter, Hugh B. Freeman, 
Report 495, U. S. Navy, Bureau of Ships, David W. Taylor 
Model Basin, November 1942. Div. 6-55 1-M6 

26. The Measurement of the Absolute Efficiency of Hydro- 
phones, Edward Cerjuoy, Ralph C. Maninger, NDRC 6.1- 
sr20-553. Report G12/101, CUDWR-NLL, Dec. 4, 1942. 

Div. 6-552-M4 

27. Motional Impedance Analysis of Undenvater Sound De- 
vices, Frank H. Graham, Eginhard Dietze, OSRD 1078, 
NDRC C4 -81-20-591, USRL, Dec. 5, 1942. Div. 6-55 1-M 7 

28. The Relation Beticeen the Absolute Efficiency of a Hydro- 
phone and Its Thermal Xoise Level, Eginhard Dietze, 
OSRD 1086, NDRC C4-sr20-593, USRL, Dec. 11, 1942. 

Div. 6-552-M5 


MIT-USL Massachusetts Institute of Technology Underwater 
Sound Laboratory. 

UCDWR University of California Division of War Research at 
the U. S. Navy Radio and Sound Laboratory. 

USRL Underwater Sound Reference Laboratories of Columbia 
University Division of War Research. 


167 


168 


BIBLIOGRAPHY 


29. Reflections from a Tieo Foot Diameter Steel Sphere, Egin- 
hard Dietze, NDRC 6.1-si20-598, IJSRL, Jan. 1, 1943. 

Div. 6-553.3-M4 

30. Measurements of Hydrophone Sensitivity Dependence on 
Static Pressure, Report Series A-1, No. 8, MIT Research 
Project DIG 5985, MIT-USL, Jan. 9, 1943. Div. 6-552-MG 

31. Loio Frequency Calibration Technique for Underwater 
Sound Instruments, Leslie L. Foldy, OSRD 1184, NDRC 

6.1- sr20-600, USRL, Jan. 26, 1943. Div. 6-551-M8 

32. Vertical Beam Pattern Measurements by Pulse Method, 

Erhard Hartmann, OSRD 1225, NDRC 6.1-sr20-605, USRL, 
Feb. 12, 1943. Div. 6-552-M7 

33. The Measurement of the Working Absolute Efficiency of 
Hydrophones, Edward Gerjuoy, Ralph C. Maninger, NDRC 

6.1- sr20-647, Report G12/166, CUDWR-NLL, Feb. 18, 1943. 

Div. 6-552-M8 

34. Acoustic Tank, Case 37866-1, Arthur C. Keller, Report 

2210, BTL, Mar. 8, 1943. Div. 6-553.4-Ml 

35. Measuring Tank Suitable for Acoustic Measurements in 

Water, OSRD 1415, NDRC 6.1-NDRC-836, BTL, Mar. 31, 
1943. Div. 6-553.4-M2 

36. Condenser Hydrophone for Frequencies Beloxv 75 Cycles 
per Second, Earle C. Gregg, Jr., Report Series A-1, No. 12, 
MIT Research Project DIC 5985, MIT-USL, Apr. 1, 1943. 

Div. 6-553.1-M4 

37. Hydrophonic Calibration; Development of Technique and 

Facilities, Reginald L. Jones, NDRC 6.1-sr212-839, BTL, 
Apr. 15, 1943. Div. 6-552-M9 

38. Directivity Considerations for Echo-Ranging Projectors, 

Eginhard Dietze, Leslie L. Foldy, OSRD 1387, NDRC 6.1- 
sr20-6l7, USRL, Apr. 30, 1943. Div. 6-551 -M9 

39. Theory of Passive Linear Electromechanical Transducers, 

Leslie L. Foldy, OSRD 1552, NDRC 6.1-sr20-878, USRL, 
June 9, 1943. Div. 6-551 -M 10 

40. Advantages of Aluminum as a Dome Material — Theoretical 

Study, Leslie L. Foldy, OSRD 1573, NDRC 6.1-sr20-882, 
USRL, June 23, 1943. Div. 6-555-M6 

41. Measurements of AX-48 Hydrophones. A Discussion of the 
Validity of Barge Calibrating Technique, Edward Gerjuoy, 
Report G12/439, CUDWR-NLL, July 20, 1943. 

Div. 6-552-MlO 

42. A Practical Dictionary of Underwater Acoustical Devices, 
OSRD 772, NDRC 6.1 -sr20-889, USRL, July 27, 1943. 

Div. 6-554- M 28 

43. Comparison of Directive Patterns of Hydrophones, Arthur 
L. Thuras, Report G12/488, CUDWR-NLL, Aug. 28, 1943. 

Div. 6-554-M30 

44. Projector Patterns of QC-Type Echo-Ranging Equipment 
on Board the USS Sardonyx, Robert J. Callen, Report 
P29/A24a/531, CUDWR-NLL, Sept. 28, 1943. 

Div. 6-554.1 -Ml 

45. Thermocouple Wattmeter (Memorandum), Earle C. Gregg, 

Jr., USRL, Nov. 23, 1943. Div. 6-553-M4 


46. Automatic Frequency Response Recorder, ,\lfred K. Latum, 
Report P35/671, CUDWR-NLL, Dec. 22, 1943. 

Div. 6-553.5-Ml 

47. Response Characteristics of Interphone Equipment (.4-11, 
Headphones, Response), Leo 1. Beranek, OSRD 3105, Har- 
vard University, Cruft Laboratory, Jan. 1, 1944. 

Div. 6-553-M5 

47a. Ibid., p. 15. 

48. Acoustic Properties of Domes, Part I, Henry PrimakolF, 
OSRD 3159, NDRC 6.1-srl 130-1 197, USRL, Jan. 5, 1944. 

Div. 6-555-M16 

49. A Low Frequency Hydrophone Calibration System, OSRD 
3311, NDRC 6.1-sr783-1308, BTL, Jan. 15, 1944. 

Div. 6-553-M6 

50. Thermocouple Wattmeters Using Unmatched Thermo- 

couples, Earle C. Gregg, Jr., OSRD 3215, NDRC 6.1-srl 130- 
1360, USRL, Jan. 21, 1944. Div. 6-553-M7 

51. Split Projector Test Unit; Description and Operating In- 

structions, OSRD 3300, NDRC 6.1-sr287-1349, HLJSL, Jan. 
31,1944. Div.6-553-M8 

52. The Acoustic Properties of Domes, Part 11, Henry Prima- 

koff, OSRD 3372, NDRC 6.1-srl 130-1366, USRL, Feb. 18, 
1944. Div. 6-555-M17 

53. Hydrophone Preamplifier for 14 HOI, Mark Harrison, Re- 
port P34/788, CUDWR-NLL, Mar. 9, 1944. 

Div. 6-554-M34 

54. Propagation of Sound Through a Liquid Containing 
Bubbles, Part I, Oeneral Theory, Leslie L. Foldy, OSRD 
3601, NDRC 6.1-srl 130-1378, USRL, Apr. 25, 1944. 

Div. 6-540.22-M2 

55. Production Testing of Projectors, Erwin F. Shrader, OSRD 
3798, NDRC 6.1-srn30-1622, USRL, May 22, 1944. 

Div. 6-552-Mll 

56. Propagation of Sound Through a Liquid Containing 
Bubbles, Part 11, Experimental Results and Theoretical 
Interpretation, Edwin L. Carstensen, Leslie L. Foldy, OSRD 
3872, NDRC 6.1-srl 130-1629, USRL, June 23, 1944. 

Div. 6-540.3-M4 

57. A Calibration System in the Loxver Megacycle Range, OSRD 
4292, NDRC 6.Lsr783-1697, BTL, Aug. 17, 1944." 

Div. 6-552-M12 

58. The Use of the Henrici Harmonic Analyser to Obtain Fre- 
quency Spectra of Pulses, l.eslie L. Foldy, OSRD 4270, 
NDRC 6.1-srl 130-1831, USRL, Sept. 18, 1944. 

Div. 6-552-M13 

59. Measurement of Projector and Hydrophone Performance- 
Definitions and Terms, Eginhard Dietze, OSRD 4245, 
NDRC 6.1-srl 130-1833, USRL, Sept. 19, 1944. 

Div. 6-551 -Ml 2 

60. Use of Models as a Tool in the Study of Underxvater Acous- 

tics, Eginhard Dietze, Joseph B. Keller, NDRC 6.1-srl 130- 
1976, USRL, Dec. 1, 1944. Div. 6-551 -Ml 3 

61. Description of the Equipment Used in the Measuremexit of 
Underxvater Acoustic Transients, Earle C. Gregg, Jr., OSRD 
4515, NDRC 6.1-srl 130-1978, USRL, Dec. 18, 1944. 

Div. 6-553-M9 


BIBLIOGRAPHY 


169 


62. Toll Systems; Trommission Measuring. 31 -A Transmission 

Measuring Set (Memorandum), Estill I. Green, Serial No. 
CD-59120-01, BI L, Jan. 1, 1945. Div. 6-552-M16 

63. Calibration of 54" Dome u'ith Experimental NRL Rubber 

Window, Eginhard Dietze, OSRD 4578, NDRC 6.1-srll30- 
1982, USRL, Jan. 3, 1945. Div. 6-555-M26 

64. The Dependence of the Operational Efficacy of Echo-Rang- 

ing Gear on its Physical Characteristics, Henry Primakofi, 
Martin J. Klein, OSRD 4859, NDRC 6.1 -srl 1 30-2141 , USRL, 
Mar. 15, 1945. Div. 6-551 -M14 

64a. /5/f/., pp. 145-147. 

65. Absorption of Coated Steel Plates, Eginhard Dietze, NDRC 

6.1 -srl 130-2146, USRL, Apr. 5, 1945. Div. 6-5.52-M17 

66. Recording Wattmeter, Earle C. Gregg, Jr., OSRD 5007, 
NDRC 6d-srl 130-2147, USRL, Apr. is, 1945. 

Div. 6-553-MlO 

67. Acoustical Calibrations and Measurements at the New Lon- 

don Laboratory , David \S . \^an Lennep, Ralph C. Maninger, 
NDRC 6.1 -srl 128-2214, Report P34/1244, CUDWR-NLL, 
Apr. 30, 1945. Div. 6-552-M18 

68. The Acoustic Shielding Effect of Baffles, Joseph B. Keller, 
Martin J. Klein, Henry Primakoff, OSRD 5408, NDRC 
6.1 -srl 130-2299, USRL, July 13, 1945. Div. 6-552-M19 

69. Transmission Circuits for Telephonic Communication, 
K. S. Johnson, Lancaster Press, New York, N. V., 1924, ji. 83. 

70. “Thermal Agitation of Electric Charge in Coiuluctors,” 
Harry Ny(|uist, The Physical Review, July 1928. 


71. “Reciprocity in Electromagnetic, Mechanical, Acoustical, 
and Interconnected Systems,” Stuart Ballantine, Proceed- 
ings of the Institute of Radio Engineers, \'ol. 17, No. 6, 
1929, p. 929. 

72. “On Interference Eliminatipn with the Warble Tone,” 
W. R. Barrow, Journal of the Aeronautical Society of 
America, \’ol. 3, 1932, p. 562. 

73. Loud Speakers, N. W. McLachlan, Oxford Press, 1934. 

74. Vibration and Sound, P. M. Morse, McGraw-Hill Book Co., 
Inc., New York, N.Y., 1936. 

75. The Theory of Sound, Lord Rayleigh, MacMillan Co., 
\’ol. 1, New' York, N. Y., 1937. 

76. Bell Telephone Laboratories Record, Vol. 16, April 1938. 

77. “Absolute Measurement of Sound without a Primary Stand- 
ard,” W. R. MacLean, Journal of the Acoustical Society of 
America, \'ol. 12, July 1940, p. 140. 

78. The Theory of Sound, Lord Rayleigh, MacMillan Co., 
\'ol. 2, New York, N. Y., 1940. 

79. “Analysis of Pulses by Means of the Harmonic Analyzer,” 
Robert S. Shankland, Journal of the Acoustical Society of 
America, Vol. 12, 1941, p. 383. 

80. Electromechanical Transducers and IVave Filters, W. P. 
Mason, D. \4ni Nostrand Co., Inc., New York, N. Y., 1942. 

81. Elements of Acoustical Engineering, H. F. Olson, I). \'an 
Nostrand Co., Inc., New York, N. Y., 1942. 

82. Electric Circuits, Members of the Staff of the Department 
of Electrical Engineering, the Massachusetts Institute of 
Technology, John Wiley & Sons, Inc., New York, N. Y., 19^3, 
p. 321. 


CONTRACT NUMRERS, CONTRACTORS, AND SUBJECT OF CONTRACTS 


Contract 

Numbers 

Name and Address 
of Contractor 

Subject 

OEMsi-212 

Western Electric Company (for Bell 
Telephone Laboratories, Inc.) 

120 Broadway, New York, N. Y. 

Studies and experimental investigations in connection 
with the development, construction and calibration 
of hydrophonic standard receivers and projectors and 
establish and operate field stations necessary for the 
maintenance of a calibration system. 

OEMsi-20 

The Trustees of Columbia University 
in the City of New York 

New York 27, New York 

Studies and investigations and the development of 
methods and equipment pertaining to submarijie 
warfare. 

OEMsi-1130 

The Trustees of Columbia University 
in the City of New York 

New \'ork 27, New York 

Studies and experimental investigations in connection 
with the testing and calibration of acoustic devices 
including operations of underwater sound reference 
test laboratories. 

OEMsr-783 

Western Electric Company (for Bell 
Telephone Laboratories, Inc.) 

120 Broadway, New York, N.Y. 

Studies and investigations in connection with the de- 
velopment of calibration devices and methods in the 
fields of hydrophonics, etc. 

OEMsr-1189 

AVestern Electric Company (for Bell 
Telephone I.aboratories, Inc.) 

120 Broadway, New York, N.Y. 

Manufacture, stocking and repair of hydrophonic ap- 
paratus. ^ 


1/U 


SKRVICE PROJECT NUMBERS 


I'he projects listed below were transmitted to the Executive Secretary, 
NDRC, from the Navy Department through the Ollice of Research 
and Inventions (formerly the Coordinator of Research and Develop- 
ment), Navy Department. 


Serx'ice Project Xuinher 

Subject 

' 

NS- 139 

resting and calibrating facilities 


NS- 182 

Projector requirements and test limits 



171 


HYDROPHONE ADVISORY COMMITTEE 


The Hydrophone Advisory Committee was the name which soon came to be used for the Committee on 
Standards and Calibration appointed by the Coordinator of Research and Development, April 1942, for the 
following purpose: to assist in establishing calibration techniques, reference levels, and standard definitions 
and terms to be used generally by all groups making underwater sound measurements of interest to the 
Navy, 

Shortly after the organization of this committee, Dr. Robert S. Shankland was selected to be its chairman. 
While from time to time the personnel of the committee changed, in general the following organizations 
were represented at meetings and were otherwise active: 

Office of the Coordinator of Research and Development (now Office of Research and Inventions) 

Bureau of Ships (940) 

Naval Ordnance Laboratory 
Naval Research Laboratory 
Division 6: 

Columbia University Division of War Research at the U. S. Navy Underwater Sound Laboratory, 
Harv ard Underwater Sound Laboratory, Massachusetts Institute of Technology Underwater Sound 
Laboratory, University of California Division of War Research at the U. S. Navy Radio and Sound 
Laboratory, Underwater Sound Reference Laboratories of Columbia University Division of War 
Research 

Bell Telephone Laboratories, Inc. 

Brush Development Company 
Radio Corporation of America 
Submarine Signal Company 


172 


INDEX 


I'he subject indexes of all SI R volumes are combined in a master index printed in a separate volume. 
For access to the index volume consult the Army or Navy Agency listed on the reverse o^ the half-title page. 


Ahsorhing paint, 122 
.\coustic axis, transducer, 21, 35 
.\coustic center, transducer, 13, 63 
.\coustic impedance, transducer, 12 
.\coustic lens, 1 1 1 
.Acoustic materials testing, 100 
.Acoustic measurements, theory 
see Calibration theory, transducer 
■Acoustic power of transducer, theory, 14 
.Acoustic pressure measurement, 58, 60 
.Acoustic radius, transducer, 31, 155 
.Acoustic screens, 45 
.Acoustic tests and equipment 

see Mountain Lakes Test Station; 
Orlando Test Station 
.Air calibration of transducers, 59 
.Amplifier circuits, 72, 74, 75, 84, 104, 
106, 118, 119, 124 

.Automatic volume control circuit, 104 
.Available power, transducer, 20, 35, 114 

Baffles, acoustic, 44 
bubble screens, 163 
diffraction effects, 157 
transmission loss, 99, 157 
Band-pass filter, variable, 137 
Barge system, Orlando Test Station, 
133 

BDI transducer, .symmetry tests, 96 
Beat frequency oscillator, 71 
Blocked impedance, transducer, 64 
Broad-band transducer, calibration, 66 
Bubble screens, 163 
Bubble-layer paint, 122 
Buffer-amplifier circuit, 76 

Calibration computations, transducer, 
138-148 

current charts, 146 
log sheets, 138 
receiving charts, 138 
reciprocity constant charts, 145 
transmitting charts, 143 
Calibration etjuipment, recommenda- 
tions for improvement, 134-137 
absorbing materials, 137 
directivity index measurement, 137 
phase measurement, 136 
pulse recorder, 137 
recording impedance bridge, 137 
transducer response equalizer, 135 
transient analyzer, 136 
variable band-pa.ss filter, 137 


Calibration techniejues, transducer 
BDI transducer symmetry, 96 
conversion of air calibrations, 59 
coiqjling, 94 

directivity pattern, 95, 97, 150 
impedance, 64, 97, 150 
low frequency calibrations, 115-119 
motional impedance method, 64 
noise, 94 

proximity corrections, 52-58 
reciprocity method, 98 
reflection effects, 40-50, 150 
response, 94, 97 

sound-pressure measurements, 58, 60 
standard transducers, 65 
testing site requirements, 37-40 
use of acoustic tanks, 40, 109, 120, 150, 
151 

wide-hand calibration, 66 
Calibration theory, transducer, 18-37 
directivity, 21-26, 35 
projector efficiency, 26 
receiving response, 27, 35 
relations between characteristics, 31- 
33 

selectivity, 27 
threshold pressure, 28-30 
transmitting re.sponse, 20, 35 
Capacitance bridge, high frequency, 126 
Charts for transducer calibration, 138- 
146 

Circuits, electronic 

amplifier, 72, 74, 75, 84, 104, 106, 118, 
119, 124 
A VC, 104 
detector, 76, 106 
filters, 84 
gating, 81 

impedance bridge, 125 
modulator, 82 
noise generator, 107 
phase bridge, 1 36 
power supply, 107 
pulse rectifier, 81 
lecorder, 78 

signal generator, 71, 81, 104, 127 
transducer coupling, 74, 105, 124 
transmission measuring, 84 
Computations for transducer calibra- 
tion, 138-148 

Condenser-type hydroj)hone, 60 

Converter circuit, 77 

Coujjling, theory of transducer, 11 


! Coiqjling circuits, transducer, 74, 105, 
124 

Coupling constant, transducer, 11, 14 
Crest factor, acoustic wave, 67, 101 
Crystal filter, 76 

Crystal transducer, charge on, 28 
Current charts, transducer calibration, 
146 

Detector circuits, 76, 106 
Diaphragm motion, theory, 16 
Diffraction around baffles, 157 
Dipole source, directive pattern, 41 
Directional sources used in transducer 
calibration, 41 

Directivity factor, transducer, 15 
Directivity index, transducer 

computation from patterns, 21, 64, 
144 

desirability for high directivity, 9 
direct measuring device, 137 
Directivity patterns, theory, 7, 41 
Directivity patterns, theory of measure- 
ment, 21, 35 

Dome design, transducer, 153-156 
Dome tests, transducer, 99 

Echo-ranging system tests in lakes, 103 
Effective area, transducer, 16, 31 
Efficiency, theory of transducer, 14, 26 
Electrical impetlance, effective trans- 
ducer, 1 f 

Electroacoustic trausducer theory, 10-17 
Electromechanical transducer theory, 10 
Electronic switch, 127 
Equipment, acoustic test 
see Mountain Lakes Test Station; 
Orlando Test Station 
Equivalent circuit for transducer, 27 

Eilters, crystal, 76 
Filters, electrical, 84, 137 
Focussing in acoustic tanks, 1 1 1 
Formulas 

acoustic pressure-gradient, 52 
baffle transmission loss, 157 
connections between transducer char- 
acteristics, 31-33 
diffraction, circular baffle, 157 
directivity patterns, 53 
dome reflection coefficient, 155 
dome transmission loss, 154 
interference pattern, 40, 50 


173 


174 


INDEX 


Frequency response calibration, trans- 
ducer, 66 

Frequency scaling, 114 
Fungicide for water, 109 

Gating circuit, 81 

Generator circuits, signal, 71, 81, 104, 
107, 127 

Green’s function, 12 

Harmonic analyzer, 125 
High-frequency instruments, 126 
High-frequency transducer tests, 104- 
115 

High-power amplifier, 119 
High-power transducer tests, 119 
Hydrophone 
see Transducer 

• 

Impedance, transducer 
acoustic, 12 
blocked, 11, 64 

calibration techniques, 64, 97, 150 
coupling circuits, 74, 105, 124 
effective electrical, 11 
mechanical, 1 1 
motional, 12 
radiation, 11, 16-17, 59 
terminating, 1 1 
theory, 11, 36 

Impedance bridges, 125, 137 
Instruments, high-frequency 
capacitance bridge, 126 
peak voltmeter, 126 
Interference pattern formulas, 40, 50 
Interference-elimination in acoustic 
testing, 40-50 

Lakes as acoustic test stations, 39 
Lenses, acoustic, 44, 111 
Level recorder, polar acoustic, 80 
Line source, directive pattern, 42 
Listening system tactics, sonar, 9 
Load impedance, transducer, 11 
Load-run observations, transducer, 97 
Log sheets, transducer calibration, 138 
Low-fretjuency tests, transducer, 115- 
119 

Materials tests, acoustic, 100 
Mechanical impedance, transducer, 11 
Meters, electrical test, 126, 127 
Models, use in acoustic tests, 114 
Modulator circuits, 82 
Motional impedance, transducer, 12 
Mountain Lakes Test Station, 68-129 
Mountain Lakes Fest Station, electrical 
ecjuipment, 71-84 

amplifiers, 72, 74, 75, 84, 104, 106, 118, 
119, 124 


co-axial lines, 73, 107 
detector circuits, 76, 106 
filters, 84 

impedance bridges, 125 
modulator circuits, 82 
noise generator, 107 
power supplies, 107 
recommendations for improvement, 
134-137 

recorders, 78, 84, 107 

signal generator, 71, 81, 104, 127 

test meters, 126, 127 

transducer couplings, 74, 105, 124 

transmission measuring set, 84 

tuning indicator, 77 

Mountain Lakes Test Station, mechani- 
cal equipment, 84-90 

Mountain Lakes Test Station, site 
characteristics, 68 

Mountain Lakes Test Station, test sys- 
tems 

high-frequency test system, 104-115 
high-power test system, 119 
low-frequency test system, 115-119 
preparations for test runs, 88-94 
test systems No. 1 and 2; 71-88 
testing tanks, 109, 120 

Mountain Lakes Test Station, types of 
tests 

analysis of transients, 123 

baffle studies, 99 

BDI transducer, symmetry, 96 

dome studies, 99 

echo- ranging systems, 103 

frequency scaling, 114 

materials studies, 100 

signal generator tests, 100 

transducer coupling, 94 

transducer directivity, 95, 97 

transducer impedance, 97 

transducer noise, 94 

transducer reciprocity calibration, 98 

transducer response, 94, 97 

Noise generator, electronic, 107 

Open-circuit voltage, transducer, 28 

Optical measurement of acoustic pres- 
sure, 60 

Orlando Test Station 
barge system, 133 
high-power amplifier, 119 
mechanical facilities, 129 
recommendations for improvement, 
134-137 
recorder, 132 

Oscillator, beat-frequency, 71 

Paint, absorbing, 122 

Phase bridge, 1 36 


Phase measurements, acoustic, 136 
Piston source, directive pattern, 41 
Polar acoustic level recorder, 80 
Power output of transducer, theory, 16 
Power siq^plies, 107 
Pressure gradient formulas, 52 
Pressure measurement, acoustic, 58, 60 
Production testing of transducers, 149- 
152 

Projector, acoustic 
see Transducer 

Proximity corrections in transducer 
calibration, 52-58 
Pulse generator circuit, 81 
Pulse rectifier circuit, 81 

() of transducer, 27, 47 

Radiation impedance, transducer, 11, 
16-17 
in air, 59 

Radiation pressure measurement, 58, 
61 

Radius, transducer acoustic, 31 
Rayleigh disk, 58, 60 
Receiving charts, transducer, 138 
Receiving efficiency of transducer, 
theory, 16 

Receiving sensitivity, transducer, 13, 27, 
35 

Reciprocity calibration of transducer, 
61-62 

Reciprocity constant charts, 115 
Reciprocity theorem for transducers, 14, 
31 

Recorders, acoustic level, 78, 106, 132, 
137 

Rectifier circuit, pulse, 81 
Reflection coefficient of dome, 155 
Reflections, effect on transducer cali- 
bration, 40-50 

correcting for reflection, 48-50 
interference pattern formulas, 40, 50 
use of directional sources, 41 
use of frequency variation, 46 
use of pulse methods, 47 
use of screens, 44 
Relaxation oscillator, 81 
Response of transducer, theory, 12, 20, 
27, 35 

Response equalizer, transducer, 135 
Rigging, definition, 38 
Rotator mechanism. Mountain Lakes, 
85 

Scale model testing, 114 
Screens, acoustic, 44, 86 
Selectivity of transducer, theory, 27 
Series connection of transducers, theory, 
14 


INDEX 


175 


Shielding of circuits, 108 
Signal generator circuits, 71, 81, 104, 
107, 127 

Signal generator perfonnance testing, 
101 

Site retpii remen ts for acoustic calibra- 
tions, 37-40 
Sono buoy, 9 

Sound-pressure measurement, 58, 60 
Spherical wave correction chart, 141 
Stjuare-wave generator, 127 
Standard hydrophones and projectors, 
1,58,65 

Stiff transducer, calibration, 59 
Stiffness of acoustic calibration cham- 
ber, 116 


Tanks for acoustic tests, 40, 109, 120, 
150, 151 

Tapered transducers, 25 
Termination, transducer, 11 
Test equipment and test stations, acous- 
tic 

see Mountain Lakes Test Station; 
Orlando Lest Station 
I’esting, transducer 

see Calibration techniques, transduc- 
er; Calibration theory, transducer 
I heory of acoustic calibrations 
see Calibration theory, transducer 


Lheory of electroacoustic transducers, 
10-17 

riiermal noise, 16, 28 
use for transducer calibration, 46 
thermocouple voltmeter, 126 
Thermocoiqjle wattmeter, 127 
'threshold pressure, transducer, 16, 28- 
30 

Time constant, transducer, 48 
Time constant formulas, circuit, 47 
1 ransducer calibration 

see Calibration technicjues, transduc- 
er; Calibration theory, transducer 
Transducer calibration computations, 
138-148 

'Lransducer coupling circuits, 74, 105, 
124 

'Lransducer dome design, 153-156 
'Lransducer response equalizer, 135 
Transducer theory, electroacoustic, 10- 
17 

complex diaphragm motion, 16 
connections between characteristics, 
31-33 

coupling conditions, 1 1 
directivity, 7, 41 
efficiency, 14, 26 
etjuivalent circuit, 27 
impedances, 1 1, 36 
power output, 16 
reciprocity theorem, 14, 62 


sensitivities, 12-14 
transducers in series, 14 
Transducer theory, electromechanical, 
10 

Lransducer time constant, 48 
Transducer types, 1, 5 
Transfer constant, transducer, 11, 14 
Transients, analysis of acoustic, 123, 136 
Transition loss, transducer, 20 
Transmission lines, coaxial, 73, 107 
Transmission loss in baffles, 157 
Lransmission loss in domes, 153 
Transmission measuring, set, 84 
Transmitting charts, transducer, 143 
Transmitting sensitivity, transducer, 12, 
14, 20, 35 

Tuning indicator, 77 

USRL Test Stations 
see Mountain Lakes Test Station; 
Orlando Test Station 

Voltmeter, peak, 126 
Voltmeter, thermocoiq^le, 126 

Warbled signal for transducer calibra- 
tion, 46 

Water, fungicide for, 109 
Wattmeter, recording, 128 
Wattmeter, thermocouple, 127 
\V4de-band transducers, calibration, 66 





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